[[Deleted on accident :X Reposting!]]
This reminds me of a suggestion someone made recently.
What if a percentage of sales went to the cost of rent in the form of a tax? This way funds go directly to the base cost of upkeep with the stall from the very onset, making more prosperous stalls receive higher income sooner (once the rent is paid, full amounts are paid out from sales.)
Stalls that fail to meet the paid rent can be paid with input silver at a higher cost should they be incapable of earning or worried they won't make enough in the month's time.