So stock exchanges like super trade or options express have thresholds for the way they price trades on their platform. So for instance, say for less than 100 trades it costs $17 per trade, and then after 100 trades it costs $14. When I think about modeling this for the number of trades greater than 100 in excel, I would initially think the broker fees being in the form of $ 17*99+(#-99)*14 $ where # is obviously the number of trades, but when I did research I found it was $ 99-# $ instead, which doesn't make sense to me because that suggests somehow the broker fees decrease and eventually go negative after a certain point. Neither of these models actually seems right, so how does one explain this?
Also, why aren't the latex tags working here?