The first answer is closest to the mark!. You can readily hire a licensed electrician for $30-40 per hour. Many of these guys are right on craigslist, and would be HAPPY to make $40 cash to come out and do this for you. Think of it this way - $40 an hour is $80,000 a year. And since we all know that they aren't going to report that $40 cash - it's even more. There are LOTS of very qualified guys who need a bit of side work.
The higher costs quoted by the other guys are laughable, and just demonstrate the incredible premium that some people put for what is very SIMPLE work. The whole thing will take like perhaps 30 min to do. It's just shutting off the right breaker, and swapping out the internals. No big deal.
This does assume that a ground wire is present, to be used.