After +6 it should be 64%, after +7 it should be 32% after +8 it should be 16% after +9 it should be 8% and so on. I would not like to see lots of +10 people running everywhere, You guys should make it difficult to obtain "Over-Powered-ness". If someone would like to get this "OP-ness" you should certainly create upgrade stones for different levels of tier upgrades.
Ex: After +6 you need a +7-+8 upgrade stone that adds 10% chance of success
This should be ideal. 
I think the Donation is a little too high. I don't have a problem giving a $100 donation every once in a while, and I am not going to say this for others but I as my self a potential investor want just a bit more, sounds like greed doesn't it? Anyway...
If you offered it at a lower price you would get more sales (Especially from me ^^). If they were lower then later on when this game is alive the economy won't be high.
Not only that but if people were to be able to have a little more flexibility with the item shop points they can get, it would be a better bang for the buck.
Just my 2 cents.. Lower prices = More sales, But higher prices = fewer sales.
Just my suggestion. Maybe you could offer coupons or get a $1 off every purchase? something like that? That would be pleasurable i'll see what you have to say.
