I wouldn't want to pay more than $75/GH/s. The way difficulty is increasing and will continue to as products ship and the market trending downwards pretty quickly as individuals cash out on ASICs for more widely accepted currencies, there is a lot of risk. You have to remember that at some oint mining is creating costs in another currency as utility providers at the very least aren't accepting BTC, so cashing out is almost a necessity at some point and the currency risk associated with any business is present.