I was about to say, it would help to have more info. Specifically, most importantly the dash number, 57-x. Any special features or markings. (e.g., Bi-centinneal.) Box? Tell us more. (Quite frankly I've found most used S&Ws and especially the Colt snakes to be way overpriced.)
Today's version, with side lock, is the 57-5 and MSRP is $1,009. Buds last price was $811.
History: Introduced in 1964. Standard barrel lengths include 1.75, 4, 6 & 8-3/8. A few special ones may have other lengths. Discontinued in 1993. Re-introduced as a "classic" (with lock) in 2005.
Consulting Supica's book, I'd suspect $700 would be over the top, unless there were something special about it. Then again, if you're a .41mag nut, then $800 might be cheap. On Gun Broker all but one are starting for $700 or more and have NO bids. The well used one is at $450, with 2 bids, but no telling what the seller reserve price is.
ASIDE: My problem with the .41 Mag is that it's not that different from the .44Mag especially when you realize the .44 mag is actually... .429. What little gain in velocity and flatter trajectory you loose in mass. Inherent bullet accuracy might be a factor but I doubt if it would matter to many of us.