That really depends upon what's wrong with it, and the quality of the parts.
They usually try using all the old parts that they can.You'll often find that they basicly only replace the bearings and seals.
It's mostly the labour that really costs money, and I've seen a friend of mine do the work in a couple of hours.
However if you need new gears the price can add up quickly. But there are alternatives to consider. (used parts)
There aren't any prices that I found on this page, but this is the company that most people in BC Canada deal with. They have a troubleshooting guide that might help you
http://www.iwerearendsonly.com/