Just to give you a yard stick, right now there is one 45 caliber Seneca on Auctionarms . The seller rates it at 98%, 0 bids have been placed, and the starting price is $600.
On Gunbroker, there are two Senecas currently listed. A 36 caliber with a rusty muzzle. There are 6 bids on it , sitting on $425, with about 12 1/2 hours to end of auction.
Also, an unfired 45 caliber with 0 bids, with a opening price of $725, and a "Buy it Now" feature for $745.
Hope this helps you decide if the one you are looking at is priced right.