Question on Cache Reserve

So I am looking into adding in Cache Reserve onto my plan, but I have a tonnnn of data, and Im not sure what the price at the end of the month is going to be. If its gunna be under $15 a month I can afford that but I don't know how to calculate it before - and don't know if I can cancel in the middle of the month.

This is the current info in screenshots of what my site is doing caching wise.