Dixie Gun Works website has an extensive list of old style firearms to choose from, and by clicking on the "price" word above the list it will sort them by cost. The lowest priced longarms they have, either in percussion cap or flintlock variety are Lyman Deerstalkers and Hawken rifles. They run in the high $200 range, like say $275 to $300.
Barring that if all you want is a decoration and never shoot it, you can buy those as well. Just do a google search for "replica musket" and there will be plenty of websites that pop up and sell non-firing guns for about 150 to 200 dollars. With that you can buy everything from fake old matchlock guns to fake Russian RPGs can be bought like that. Some even have moving bolts and such, and even a few are capable of firing blank cartridges.
