Laptop Mag’s Michael A Prospero says:
Want to squeeze even more performance out of your MacBook Pro? As our resident Geeks Geek would say, then get an SSD, you putz! Adding an SSD will make your Mac boot faster, copy files in the blink of an eye, and make the system more responsive when multitasking.
For this how-to, we replaced the hard drive on a 15-inch MacBook Pro (non-Retina) from 2012 with a 480GB Mercury Extreme Pro 6G SSD from OWC, which costs $579. That works out to about $1.21 per GB. By comparison, if you bought the 512GB SSD from Apples site, that would run you $800, which is $1.56 per GB.
Fortunately, its fairly easy to replace the hard drive.
Prospero walks us throuhg the process in an illustrated tutorial here: