You'll have to keep a very keen eye on ebay and craigslist. There aren't a ton of cheap MBPs out there that can handle new, top of the line games. To be frank, I'm not quite sure there are any that can handle graphic intensive games very well at that price range.
I'm sure this won't be a popular opinion on here but if you're looking for a cheap computer that can game well, you're much better off looking at a PC.
*runs and hides*