Newer model trucks may have preinstalled hitch prep packages. In that case all the buyer has to do is drop the hitch into the hitch prep holes. No installation required.
The higher prices being mentioned in this thread are for high end 'slider hitches' which may be required if the truck has a short bed and the fifth wheel is an older model. Newer, meaning less than 8-10 years old fifth wheels probably don't need a slider hitch.
Non slider hitches run in the $500-$1000 range, with some of the better models running around $900. If the owner buys the installation package (and hitch) himself from one of the many online retailers, the price will be a lot lower. Tweetys.com and Etrailer.com are two popular sellers.