Also depends what part of the country you are in. Here in SoCal I tend so see them only in the $1500+ range. They are rare in around here too which makes it worse.
If you can hear it run and can see what oil pressure it has, and its good then yes that's a fair price. If you go to re-build one yourself count on 3K minimum for just parts and machine work for a mild build.
Couple of things - yes location, also is it factory original or rebuilt? What is the oil pressure? How many miles on it? Can you hear it run? What are your expectations? Do you think its going to give you 13 in the 1/4? Is it just a weekend cruiser or a daily? Can you afford to rebuild it if it fails? Most of our motors get little use, less than 5K miles a year. My opinion: for a well maintained 455 with 100K miles on it to go another 30K is 6 more years and a safe bet. If its low mileage and factory, good oil pressure and a trusted seller, then yes for me. I was about to pay $3K for a well running rebuilt motor with some miles on it, but didn't because the oil pressure was funky.