You can purchase some fairly decent flintlock packages these days that are nearly finished. Of course, the more you have them do for you the more expensive the kit becomes. My last kit went a little high, but then I wanted a really fine swamped barrel with round bottom button rifling, really super curly maple stock, and some modifications to a high quality lock....$700 in parts alone.
And I still had to inlet the barrel, buttplate, trigger assembly, lock assembly, lock plate, ramrod thimbles just for the stock. I also had to handcut and shape the two sight dovetails, breech the barrel, and attach the barrel lugs underneath. Those steps alone saved me $200!
A decent muzzleloader gunsmith can have your barrel cut, breeched, and drilled for a touch hole liner for about $100. Fitting in a new lock in the existing mortise? Well, there's another set of worm cans!