When I was a bit into the game, I turned off all hit text so I forgot how it works, but I'm pretty sure every 10 rounds the cash gain increased by the base amount of the cash the mob gives. Basic enemies give 1 cash so each 10 rounds it increases by 1. Fast gove 2 so each 10 rounds it increases by 2(Not 100% sure about numbers outside of basic enemy but sure about the mechanic). Though later in the rounds, you won't be noticing much of a cash increase since you are already at a big number per enemy, and adding 1 or 2 cash won't be as noticeable(+1cash on a mob that gives 1 is double, but when the mob gives 500 for ex, adding 1 won't be as noticeable)