Depends on the work done to it, you could easily have $5,000 into that rebuild, or if it was a runner that cleaned up nice with a hone it might have a fraction of that. But a complete 392 that turns over and needs to be rebuilt fetches $1500 pretty easily. Generally it's what the market will bear on these things -
Prices go all over the place, but in the end, do yourself a favor and don't buy one that's been lying out in the weather because it's "cheap". In the end, it will be the most expensive of the bunch. If someone wants big money for one, but balks at the idea of you being allowed to look inside it, pass.
If you can get the receipts showing the rebuild and what was done to it $4500 is a good price. You can get a long block from PAW last time I checked it was $5900 and you had to put it together. Also where was the machine work done and what did they do? Good luck.