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:
http://i60.tinypic.com/2lm5tup.jpg
Until now those 33 GH/s paid out 0.258 BTC
http://i61.tinypic.com/vzd4cm.jpg
33 x 0.0075 = 0.2475 BTC
Pay Out until now : 0.258 BTC
Positive ROI until now 0.011 BTC with about 4.5 Years left in contract
That's a good method. Thanks for the info.