A clean polytech is gonna run about $600.
As long as the headspace is in spec it's likely to be a good shooter, quality is roughly equivelent to SAI.
It'll cost another $100 to shim the gas cylinder,replace the trigger, hammer and hammer spring assembly with M1 garand parts(better and cheaper than M-14 parts unless you've got an autosear), put on a USGI stock, handguard and flash supressor (a lug will cost more unless you get a repro FH) for 922r compliance. Then you need to check the headspace and fit a USGI bolt in it if it's too long, that's another $150 (for a complete bolt), and maybe replace the sights and springs, but many are good as-is.
That rifle will be functionally just as good as any service grade M1A. The receiver, trigger housing, oprod, barrel, and gas system are all first-rate on the polytechs. And you'll have a set of spares.
A used M1A with commercial parts will run about $1000.
M1As with GI parts seem to be running about $1200-1500. All the springfield receivers are cast. The springfield may still need shims and troubleshooting. Occasionally you'll find one cheaper, but that's getting harder all the time now that a set of GI parts will bring $1000.
LRB makes the best, but they cost too much.