Sounds like a quest for the holy grail, tbh. In an endless game, there is no way to stop people from accumulating money (or loot, or levels, etc.). On the other hand, value of money is based on useful things one is able to exchange for it. If there is eventually no use, money has no value and thus neither selling unneeded loot...
Let's say (please pardon me for such simplification) that there is something like functional minimum of cash one needs to obtain (~still a chance to beat), then confortable average (~enough but not overabundance), and some humanly reasonable maximum reached from minmaxing/grind (would multiply that by 2 because no human reasonable).
What if the problem can be divided into two? For example, the game's main economy to be primarily balanced somewhere around the average region around the things that have impact (guessing that's the aim), what feels right for you, and avoids the feel that one needs to grind on Vigilante difficulty for the majority of players. But in the same time, can also offer additional money sinks with significantly lower impact per money paid, so no player with too much cash will be left behind, yet the balance stays mostly undisturbed.
Facilities or buying useful items like ammo or crafting stuff - guess that would fall into the first category (unless you swim in ammo). The second one can either have things of low impact (handy if repeatable or with diminishing returns, eg. paying for a district intel %); or it can be also akin Ferrari. For example, if somebody manages to accumulate 100k Euro the game offers an option to pay off the Church CEO. A sort of getting a hidden alternative ending as a nod to the greedy playthrough. Those are just examples though.
The main point is that maybe it's not needed to be balanced around extremes as long as there is candy with nonzero sugar in it.
Btw. there is one handy money sink in the game already. You can buy levels from Cuda
