check this place out, I haven't had them do any mastering for me, Although ^^^^^Urple.Eeple has and I was happy with the way my track turned out!
I heard about this place from reading the comment section on MartyPartys facebook. Somebody asked about mastering and He replied by saying it cost about 40 quid which is about 65 US dollars (googled that...)
they don't have any prices on the website, so I am just going with what Marty said.
I also looked at the client list and I recognized Aphex Twin and Flux Pavillion on there...