Now you're talking! That's a quality arm, neither a cheapie nor a rarity. You'll be proud to own it and it will serve you well, even as it's worth some money to him.
New, they're pushing $800 today, but used you see them in good shape everywhere from $300 to $450 depending on the region. Online I'd be jumping on it at the lower end of that range, but slow to pull the trigger at the upper end. But holding it in my hands here in town, I'd expect to pay toward the top end due to 54 cal rather than 50, no shipping, and the chance to look it over real close.
Hat's off for your concern that the deal be fair to your friend too. Not enough of that going around! :hatsoff: