What a coincidence
I examined a rifle very much like the one you posted while on vacation. It was not for sale. It was owned by a former re-enactor (Gave it up due to bad hips.) who owned the B&B we stayed in.
It was made sometime around 1967, a Navy Arms, Zoli. I thought the fit and finish on the rifle I examined was not outstanding. It was not well cared for and had suffered a bit from neglect. Still very shootable and easily could be brought back to good condition considering it is not a collectable piece.
I am not up on the values of muskets and rifles but I think (depending upon what kind of friend the person is), I would start a little lower and let him work up to 200.00. As a cautionary comment, that strategy invites the situation in which the friend feels compelled to take your first offer even though he thinks (or worse yet, discovers at a later time) that he is being taken for a ride.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson