per day: 0.00451612
per week: 0.03258064
per month: 0.14225806
You initial pledge of ONE bitcoin will be paid back within a maximum of 7 month
You are in essence asking for a loan of 1 BTC and paying back ~28% extra in 9 months (or even a bit more) - but only "if the Mini Rig runs as great as they say". These are far worse conditions than most loans made to random strangers on this forum. Why sould I trust you - for 9 months?!
Also you don't seem to know a lot about mining, if you say "Running 25GH/s would [...] make transactions of WalletBit so much quicker."
Sorry, maybe try to open up an asset (with verification!) on GLBSE, it's made for these things (support for dividends/paybacks, easy handling of shares...). The idea is great, but quite a few people also had it already - and they are expanding aggresively.
Good question about why you should trust me, and to be honest I cannot provide an answer, other then I keep bitcoins safe for people at WalletBit. This is the best I can do to prove my honesty.