As a point of comparison, I paid $300 for a 750 GB Samsung 840 EVO on Newegg last Feb (it MAY have been on sale at the time. That's an almost $80 savings compared to the 500 GB 850 PRO on OWC. Newegg currently has the 512 GB 850 Pro for $257, over $100 cheaper than OWC is asking.
I can't speak for the installation in a 2011 directly, but if it's anything like the 2012 installation in my 2012 was pretty easy. I took my time and followed the step by step on iFixit. You can get the kit from them for $30 with all the tools, screws and the cable needed.