I have a MacBook Pro (MacBookPro5,2 / 2.8 GHz) with 4 GB of ram (2x2GB). I'm looking to upgrade to 8GB. The memory in it now is DDR3 PC3-8500 1067. Checking out prices for 4 GB sticks of PC3-8500 there is about $100 difference for "apple certified" ram. Will any DDR3 PC3-8500 module work or is there really a difference?
The one caveat I would add is that with Apple, don't deviate from the specs.
My early 2008 MacBook takes 667MHz DDR2. I upgraded from the 2x1GB that it came with to 2x2GB of Crucial 667MHz DDR2 and never had a problem.
With PC's I've never had a problem using faster RAM. However, when I once tried installing 2x2GB of Corsair 800MHz DDR2 in my MacBook it wouldn't even boot. Apparently Macs can be very anal. Go figure.
So FWIW my advice is that while you don't need to pay for special "apple approved" RAM, it is probably prudent to still be conservative. Get RAM from one of the major manufacturers with specs which exactly match what Apple says your MacBook Pro uses.
Also a good idea to run at least the Apple Hardware Test after installing the new RAM just to sanity check it. (I'd actually try to boot some flavor of MemTest and let it run a few passes, but maybe that's just me).
Macs use conventional PC Laptop memory, despite what Apple says. I just had to send mine in about 5 months ago with 3rd party memory installed, and they returned it with the memory in a separate package, but otherwise didn't care. It's just like buying generic replacement ink cartridges or generic replacement laptop batteries.