I would shop around. Don't just pick someone out of the phone book. Tour their facility, eyeball what they have, and listen to things that they have already done.
You could certainly try to do it yourself, but a lot of the mastering houses use eqipment most of us would not be able to imagine (or ever afford).
I've heard things mastered very well for under 1k, but I've also heard things that cost three times as much sound like crap.
Check around.