Customized machines are a little tougher to price then stock. Refurb store is selling this machine for $1169, you've upgraded the RAM from the stock 4GB (+$100) and gone 7200RPM over stock 5400RPM(+$20). I don't know what, if any, warranty or AppleCare you have left, the refurb comes with 1 year, so, deduct some if you less than a year. Also, unless you did a CTO to get an Apple 7200 drive, the hard disk isn't covered by Apple warranty anymore. Taking all that in to consideration, I'd say $1200-1275. The case adds little value as almost everyone has their own taste. Personally, I'd put the original 8GB back in to sell.
Apple Certified Macintosh Technician (ACMT) / Support Professional (ACSP)
MacBook Air (13-inch, Mid 2012) 8GB RAM, 256GB Flash Storage
Mac mini (Late 2012) 16GB RAM, Fusion Drive (128GB SSD/750GB 7200RPM)
iPad mini 16GB, iPhone 4S 16GB