Wow, that's a cool pistol.
I'm not sure I can help in valuing it, but I can give few example sales below. With antiques the value tends to be whatever the buyer is willing to pay at a given moment.
Also, the clarity of markings, the condition of the item (including in the bore if one intends to shoot it). The state of the lock internals etc. All have impact on value. So please take these examples as just that examples. Not a value estimate of your pistol.
I saw percussion converted pistols of this and later models sell for $330~$850 at auctions.
Here is one auction in 2018 where two of model 1777 were sold for $1495
Two French Model 1777 Pistols | Rock Island Auction
In the auction guide they estimated value of the pair to $1300~$1900 so probably back then they would estimate half that for one.
However, people often get carried away during auctions specially when there is more than one bidder. It is perfectly possible for pistols like that to be sold much cheaper if there is not much interest. For example here is one sale from 2018 for €280 ($330)
France - Saint Etienne - Model 1777 - Cavalry - Percussion - Pistol - 17.1 mm
Personally I'm not looking for one at the moment. The best value estimation there is is someone saying "yes, I will pay this much for it"