100% legal to own and possess. You should be able to have that pistol deregistered as it's almost certainly an antique. You can do that buy calling the CFP and sending a firearms tech some pictures. There should be proof marks on the rear face of the cylinder and maybe the bottom of the barrel. Lefaucheux was a major producer of pinfire guns, paving the way for a lot of innovation in metallic cased cartridges in Europe. There are also an enormous number of Belgian gunsmiths in and around Liege who copied the Lefaucheux design and produced them in all sorts of pinfire calibres. Pricing would depend on how well it locks up, bore and cylinder condition and who made it. For a functional, slightly grubby revolver $300-$450 would be reasonable. They're easier to load for than the later rimfire revolvers but are still quite niche. I would suggest that $500-$700 would require something a bit special, a military connection, significant engraving, high condition or a renowned or rare manufacturer.