What Haggis says is true, but for a person who hasn't built a custom rifle little things like drilling the ramrod hole can be a real challenge.
It might be worth a few bucks to the novice to have that drilled and it doesn't hurt to have them cut the barrel channel. For someone unfamilure with lock placement (relateive to the barrel) it doesn't hurt to have that roughed in too.
All of the semi-finished stocks I've built from had enough wood on them to still require wood removal to take it from a klunky course look to a nice finished shape.
Sooooo...Yes some of this costs money. What you have to do is decide how much you know about these things balanced aginst how much do you want to pay.
Before I bought from TOW, I would at least get a catalog ($3.00) from Pecatonica River Long Rifle Supply and see if they have anything of interest. Although TOW has a larger selection of styles, I think its prices are a little higher.
Muzzleloaders Builders Supply, Tennessee Valley Muzzleloaders, also offer "kits" and are worth lookin into.
Edit: For more of my thoughts on these kits see
http://www.muzzleloadingforum.com/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=11;t=000060
Edit 2: I don't know what TOW is asking but the Pecatonica catalog I just recieved today says their Full Stock Hawken kit in Flint is $426.70 plus wood. Straight#1 maple = $80, #2 maple =$100. #3 maple =$140, Walnut or Cherry = $135.