Greebe, while I wholeheartedly agree with what Stumpy and the others have said (and probably fall into the category of those who can build a $400 gun from $800 in parts), there is no greater satisfaction than taking those parts even if it's just a kit or a mixture of parts maybe including a precarve stock, and building something that you can use and enjoy. I've built 4 rifles ( 2 long rifles, a chunk gun and a halfstock) and while none will win any kind of beauty contest (they're certainly not the quality of a Brooks or a House or any other well known builder, they have won a few matches and/or gotten game. Some were made with precarves, some with rough blanks. Some big builders use precarves, completely finished locks, triggers and other parts. Nothing wrong with that. As was said earlier, colonial period smiths were importing locks from Europe and buying their barrels from barrel mills just like we do now.
If you have any mechanical skill or woodworking skill, it certainly makes a big difference. Some guys seem to be able to do anything and make it look easy. Some slobs like me know just enough to get themselves in trouble. But I for one won't give up. I'm slowly getting a little better at some things, even woodworking! I made the mule ear lock for my chunk gun and now I'm starting on a lock hand forged from wrought iron. When it's done, I'll start on another rifle with more handmade parts. Hopefully, it'll be a little more like those I've admired for 40 years.
I think I may have drifted a bit, but I'll try to get back. Custom builders also have to build what the client wants. In some cases that includes using a precarve or other part that the client wants used in the build due to a particular grain or shape they like, or if the client wants the gun within a shorter timeframe it may be best to use parts closer to the finished state. I can tell you for a fact that if a gunbuilder had to make all the parts of a gun and then put them all together, I doubt that a handful of shooters in this country could afford one. Well, I've blathered on enough.