AT&T iPhone 3GS to iPhone 4 Advice?

I currently own an iPhone 3GS, which is on AT&T. I purchased it on March 27th of last year. I kind of got screwed over since they announced the iPhone 4 in June, and were giving refunds to people who purchased a 3GS close to that announcement day. Ever since, I have wanted an iPhone 4. I recently came across this eBay program that buys your used smartphones, and because of that, I can get $200 for my iPhone. That would cover the full price of the iPhone 4, but I would need to start a new contract to get one, and we all know the hefty sum that they ask in order to cancel the current contract and start a new one. So my questions are, how much as of right now would it cost to cancel my contract and start a new one, and also, is it even worth the trouble? The iPhone 5 is set to launch in the summer, uming that it follows Apple's typical launch cycle. I found out that in August, I could upgrade to a new iPhone, staying on the same contract, for only $18! I need someone to make this decision for me, it's overwhelming!