XP = treasure spent is one of my top-shelf house rules, up there with encumbrance by strength. This method of managing advancement has become pretty important to how I play D&D. However, it presents two challenges. One is that there must be supply to match player demand (that is, interesting things to spend money on). Living costs, retainers, and equipment constitute partial supply.
Another is that it makes players more present-biased, as treasure must be disposed of in order to acquire XP, which works against saving for large expenses such as strongholds. This is not entirely a bad thing, as it incentivizes the iconic Swords & Sorcery behavior exemplified by Conan, Fafhrd, the Gray Mouser. However, this can work at cross-purposes to some degree with the mid to high level domain game.
To address these issues, I suggest introducing a new common town feature: the bank (or merchant house, or whatever you prefer to call it). PCs may deposit money in a bank to take XP immediately. Bankers, being important folks, are not going to concern themselves with pocket change. If you want to make a deposit, it must be in round numbers with at least four digits (of whatever coinage is being mapped to experience points). Withdrawals are not in coinage, but instead in bank notes (which have similarly large denominations). Spending bank notes does not grant XP (that would be double dipping). Bank notes are primarily useful for funding expenses such as mercenary companies, fortresses, and ships.
From a game perspective, all that need be tracked on the character sheet is the bank in question and the deposit amount. Of course, bankers pay interest, and this is represented by 1100 out in note value for each 1000 put in (nice round numbers, because I am lazy). This provides the standard economic trade-off between less now or more later. The amount on deposit could also double as some sort of reaction bonus for social interactions with the bank.
Needless to say, someone will bring up the bank as lucrative target. And I say to that, go for it! Heists make good adventures. However, good luck getting more than a few thousand coins from knocking over a bank, as most bank assets are in the form of obligations owed by others, not hard currency, and any heist will of course have social ramifications (not least being the adventurers becoming wanted criminals if identified). I could even see adventurers being called in to defend a bank with which they have deposits.
If you owe the bank $100 that’s your problem. If you owe the bank $100 million, that’s the bank’s problem.
— John Paul Getty (mutatis mutandis, of course).