You pay for what you get.
I've seen kit versions of the guns that big stores like Cabela sells and they're good for starters. I've also seen kits that require a lot of work and the parts aren't even quite right.
I've also seen kits that require some work but the parts are terrific. It all depends on what you want and for authenticity, you pay bigger bucks.
One thing about kits though. Even if you buy a good $700 kit that has potential to make a $5k gun, if your workmanship is amateurish, you have a $700 gun (price of parts plus no $ for labor of assembly). That same kit put into the hands of a craftsman produces a $5k gun plus.
My first BP rifle was a kit and it's a Spanish cheapie. Still, it was what was required to get me into the game of blackpowder rifle shooting and I still love that gun to this day. It was on the dealer's bargain table for $45 (1988-89).
In selecting a builder and there is no shortage of builders, find one that builds the "style" that you like. Some builders build for certain periods or copy certain guns. Others will try anything. You want someone who can build the gun you envision.
About the only kits I would buy today are Jim Chamber kits. He makes his own line of locks (Siler) and warrants them (break a part and Jim will send you a new one). If you're serious and want to take a class on building guns, Chambers is the way to go.