DISCLAIMER: THIS IS ALL PROBABLY JUST A COINCIDENCE
I've always been a bit of a numberphile. The other day, when I went to figure out just how worth it paying an upgrade was for a girl versus the fighting the upper limit of Fredy, I discovered something interesting (spoilers: if she's under five stars fight, otherwise save Kobans). Shaving off the thousands, I found that one Koban is worth about 2,731.4814...k each relative to Fredy's upper limit, or in other words, 590/216. That number looked oddly familiar to me, so I tried adding one to the denominator. There, I got about 2.7188940092165898617511520737327. That was looking VERY familiar to me, since e -a very important number in math- is about 2.7182818284590452353602874713527- less than 0.0225% of a difference. Hence, I propose that with some simple rounding errors added into the mix, the internal equation for Koband to in-money conversion is y = 1000e(x+1), wherein y is money and x is Kobans at the optimal rate of mob fighting, while x is not equal to zero.