Memory has dropped in price lately... but to be sure you're getting the correct stuff, buy (or at least verify) it through some of the memory makers' websites... crucial.com, kingston.com, etc. The $50 price sounds about right for two 4GB sticks.
Originally Posted by Jclisenby
According to my quick research, you need two 4GB sticks of 204pin PC3-8500 SO DIMMs that run at 1066mHz. And the mid-2009 MacBook Pros maxed out at 8GB.
BTW, there's a great, free reference app in the Mac App Store (also iTunes App Store) called MacTracker - info on virtually every piece of hardware that Apple ever made. Every Mac user should have it installed on their machine for reference. There's some really interesting stuff in there.