Is it at all possible to check your purchase history? As in, how much you paid for GH?
As far as i know: Nope.
I sometimes make Screenshots of current prize when i bought GH/s. Example
Back in March this year when i bought this 33 GH/s Prize was:

Until now those 33 GH/s paid out 0.258
BTC
33 x 0.0075 = 0.2475
BTCPay Out until now : 0.258
BTCPositive ROI until now 0.011
BTC with about 4.5 Years left in contract
