I was comparing the 5e book costs of a horse (50gp), cart(15gp), and trained hireling (2gp/day) to the cost of 1 influence. If we figure cost of storage, a second horse and second trained hireling, and say that these costs pay for a year (not saying to make influence a yearly cost, just thinking out loud) then we arrive at ~1500gp. Some 5e adventures may have time pass quicker than others, with years occurring in a campaign... others might take a month or two :'D
If you're wondering about costs though, since you listed Level Up A5E (I think?) in consideration, you CAN see in their Trials & Treasures book how much wealth a character should roughly accumulate on a level-by-level basis. Page 176 of T&T (their books are freely accessible on a5e.tools) "Treasure by level" says each character should have acquired ~2100gp in total by level 5. So I guess it depends on how much of an investment you intend Influence to be.
... Of course I don't follow this, I roll randomly for treasure (using their a5e tables) and I'm not even sure how much I give out relative to this chart :')
One more thing to consider re: a5e is the cost of Strongholds, listed in the Adventurer's Guide. They start at 500gp for an "Encampment," 1000gp for a 1000sq ft Guild House, etc.
It's definitely a tough thing to nail due to the abstraction- and I do appreciate the simple abstraction of the system.