Suggestions range from unforgiveably heavy-handed (theft, surprise taxes) to transparent railroading (training costs and such.) Then there are some intriguing ideas, like carousing rules, which award XP only for treasure spent on carousing between adventures. One variant I've seen (and I can't remember where, so if it's yours, step up and claim credit!) adds other options for characters who are more studious or civic-minded to gain XP by spending their gold on research or charity. Though I personally don't think I'd use it in my games, I can see the advantages it has over the other methods in terms of player agency: Players have an actual choice, to spend their hard-won loot on carousing or some other form of reputation-building activity, in effect dissipating the loot for XP, or they can save it and spend it on more practical things that aid their careers in more tangible ways.
I'm thinking of something a bit different, though. It wouldn't have to conflict with the spend-money-to-earn-XP paradigm (in fact, it would blend pretty nicely with it.) In classic console-based RPGs like Final Fantasy and (my personal favorite) the Dragon Warrior series, the heroes don't have access to the entire game world all at once. They must earn access to new places and win the favor of important people in order to advance. New areas and quests are unlocked as the heroes progress in the game. Rescue the merchant's daughter from a monster attack, and he rewards you with a ship so you can sail to new lands. Recover a magical key from a remote town so you can open all the doors in the castle. Gather enough gold so the old man can hire workers and realize his dream of building a tunnel under the mountains to the next kingdom.
The analogy of console RPG to D&D isn't perfect. The money gathered in most classic CRPGs is spent almost exclusively on weapon and armor upgrades, of which there aren't a lot in D&D. The unlocking of new areas to explore and new quests to fulfill is driven mostly by acquisition of "plot coupon" items or defeating boss enemies in previous quests, and is usually pretty linear in nature. Still, the general principle of unlocking new challenges translates quite well to a tabletop RPG sandbox.
The idea of the sandbox, of course, is that the PCs can go anywhere. That doesn't mean everything has to be free and easy, though. All sorts of barriers exist to hinder characters going wherever they want to go - barriers physical, magical, political, and social. Money can buy passage in many different ways. What's different about the sandbox as opposed to the CRPG is that where the CRPG has a fairly linear plot, the sandbox offers many paths, none of which is mandatory. The characters don't HAVE to spend their hard-earned gold on any one of them if they don't want to. Players can choose which opportunities to pursue and which not, and that makes their decision to spend gold on pursuing them meaningful.
The DM's job is to seed the campaign with enough rumors and facts about these difficult-to-reach places that the players are dying to get to them. Put the ideas in the players' heads early, long before they have the resources to actually go there, and remind them often enough to keep their imaginations churning. Make sure you keep track of what you've told them, but there's no need to develop anything in great detail until they're on the cusp of actually doing it. Sketches and hints are enough for now. Tell them how the sailors talk with superstitious reverence of the Phantom Isle rumored to be the last, cursed stronghold of an ancient race. Toss out rumors of the Lost Temple of Bara or the abandoned city of the dwarves. Show the benefits that more experienced champions earn through their connections with powerful guilds and nobles. Have a half-mad caravan guard come stumbling home, the last survivor of an ill-fated expendition to the exotic lands beyond the mountains, raving about cities of gold and jewels.
In the meantime, keep them occupied with the usual low-hanging fruit of beginning adventures - local ruins, orc raids, the abandoned mine where people hear that mysterious knocking sound at night. These need not be boring or mundane adventures, but the gleam of what's just over the horizon should lure the party ever onward and have them counting their coppers after every new haul to see if they've got enough to bankroll the Big Quest.
How do you get them to drop coin on these things?
- The most prosaic example is the island. Booking passage to coastal towns a simple matter. Getting a captain to drop you on an ordinary island slightly off his usual route might cost a bit more. Talk about an expedition to some dreaded place, and you might have to buy a ship outright and hire a crew of the craziest and most desperate souls you can find.
- The doorway into the lost temple is sealed, and the walls around it covered in inscriptions in a long-forgotten tongue explaining how to gain entrance. It's going to take a sage a few months and a big budget for old books to decipher it. Maybe the party even needs to take him on the dangerous journey to the place itself.
- The cave that once housed the nefarious gang of thieves is covered by a landslide. It'll take a party of adventurers months to clear it, unless they shell out some coin to hire laborers and keep them safe while they work.
- There's a wilderness area, rife with many ruins and dungeons to explore, but far from any town or other safe haven. The adventurers might need to construct a secure base, perhaps a pallisade fort, and garrison it with mercenaries so they have somewhere to come back and rest between delves. Otherwise, constant threat of wandering enemies will take its toll.
- Connections to the Merchants' Guild, the local nobility or royalty, or a secret society might bring lots of special commissions, but in order to cultivate those connections, you're going to need to grease the wheels and dress the part. Showing up at the Duke's Ball in full murderhobo attire is not likely to end well, let alone impress the Duke into taking the PCs into his confidence. The PCs may have to spend a lot of time and money cultivating their image, purchasing a villa or manor in which to live large and host social events of their own, before they're even invited to the Duke's events.
- On the other hand, sometimes the best friends to have are from low places. To win the trust and admiration of the peasantry, the PCs can spend their gold on temples, houses of healing, roads and bridges, fortifications against marauders, or whatever the locals most need. Especially worthwhile if one of their goals is to overthrow or otherwise undermine the authority of a local despot.
- The dwarven ruins were sealed so long ago that even the dwarves have forgotten why they were abandoned, but none is daring enough to return, despite persistent rumors of hoards of mighty weapons and armor and the forgotten secrets of their forging. In order to open the seal, a series of three keys must be made, with precious metals and stones, by the most skilled dwarven craftsmen, and they're not going to do it for free.
- Or, perhaps the current dwarf stronghold guards the only pass through the mountains to the mysterious kingdoms beyond, and the dwarves are xenophobic and suspicious of humans. The party could spend months doing favors to win their trust...or offer them a tribute of gold and jewels to gain their favor and passage through the mountains.
- In the depths of some dungeon or ruin, the party stumbles upon a magical portal. Unfortunately, it's broken, and will take a lot of resources and the assistance of NPC experts to repair. Tantalizing rumors abound as to what lies at the other end.
- The PCs find a map to the location of a sunken ship or island, and need to have a high-level magic-user cast enough Water Breathing spells for the whole party. She's going to need a good incentive to leave her own research and go on a sea voyage, or even just write up a supply of scrolls of the spell. She doesn't accept IOUs or payment on contingency, but cold hard coin will do just fine.
The range of possible expeditions is limited only by imagination. This isn't meant to be an advancement tax; the players should always have a choice not only between various quests on which to expend their money, but whether to take up any of the costly expeditions at all. Players can keep on scouring the easily accessible areas if they're determined to pinch coppers, but should be aware that the best loot and the most amazing discoveries haven't remained undiscovered by being easy to find and break into. As they say, you've got to spend money to make money (or magic, or discoveries of lost civilizations or earth-shattering knowledge of the campaign world's secrets, as the case may be.) Creative ways of reducing or avoiding monetary costs should of course be openly entertained by the DM, but sometimes spending money is just the best and most efficient way to go. If the PCs would rather bribe the dwarf king than perform a series of Fed-Ex quests for him, don't discourage them.
Of course, there really should be something awesome awaiting the PCs when they finally reach their long-dreamed-about destination. It need not be what they were expecting, or even what they were told to expect by madman or by sage, but it had better be good enough that they'll be excited about unlocking the next inaccessible place too.