99% of antique guns are put on display in museums or are owned by private collectors. The prices of said antiques range from 25,000 - 100,000 USD depending on their rarity. I'm pretty sure that it's not a worthy investment no matter how you look at it.
The current companies that produce guns only produce replicas, and those can be priced from 25 - 300 USD, those are your best bet. As I said before, Pietta sells some of them, and you can find replicas on various parts of the Internet.