In 9-12 months, paying $2500 for 50GH will be a losing proposition unless the value of BTC hits $500 US. It's going to be hard to break even when 50GHs is a tiny % of the processing power. The difficulty is about to skyrocket. The value of BTC has to rise with the difficulty or mining won't make sense. <- remember this
I think if difficulty will go up 10x or more price will definitely correlate, because most miners will just stop selling generated coins in loss and hoard them waiting for the better exchange rate.