Greetings!
This appears to be an old Vaquero (with the larger frame). These were a limited edition but I don't recall the production numbers.
As a very desirable dual cylinder model in a very desirable finish and barrel length, the price might be a little high, but not by much, maybe $50-100 if NIB. (If you can find a NIB one for $550-600, please let me know where it is!) If there's not much wear on it, try to talk them down just a bit, but don't expect much of a drop; the prices on the old model Vaqueros have gone up quite a bit over the last few years, as supply has decreased.