I've been building guns for over 25 years. Today, there are more suppliers, with more parts than ever before.
The more interested you are in authenticity, the more difficult it will be to find those parts.
These are actually good problems to have. Suppliers and manufacturers are not going to stock parts that they can't sell. So the more specific the parts, the harder they are going to be to find.
Many are the times that you must make or modify an existing part to make it more like originals.
Back to price. A custom flintlock rifle in today's world is $1500 to who knows. All depending on authenticity, and labor, as well as the builders reputation.
Parts are going to be $800 to $2000, again, depending on how period correct you want to be.
Not bad considering a rifle gun would cost several months wages in colonial days. Today that would be a whole lot of money.
You won't see builders, or suppliers, or parts manufacturers getting rich today.
$1000 worth of parts, 150 to 200 hours of labor, and the world thinks you should be able to buy it for $600.
Sorry for the rant.
I relinquish the soap box.