If its rust free and clean id say $4000 is average. Market dictates though, $4800 isnt obscene, although not a really great deal in my eyes.
I think the HP/Torque numbers are inflated as others have mentioned, but like everyone else unless he has paperwork to back it up those numbers are fiction. "My Firebird has 800 hp at the wheels!" yeah, right...
Also inflated is the fuel mileage i think. You can expect to exceed 20 mpg with proper (stock sized) tires though, but 26 is a bit much. Possible spedometer error could account for the difference.
I paid $3500 for my bone stock yet really clean 83 with a 6.2, it had a small amount of rust around the door sills but i felt i got a good deal considering the area i live in. Here in my neck of the woods people will buy anything "diesel" for outrageous prices because of the bio-diesel craze. I also bought mine when gas prices were spiraling upwards, so that didnt help.