If it is a real preban rifle, $900 is a good buy. What is the serial number? The cutoff I think is around 84,000. But you may want to write it down and contact Springfield Armory to find out if the receiver really left SA as a complete rifle before the ban started.
Another advantage of owning a preban M1A is the fact that SA used to use USGI parts back in the days. Current production M1As are often plagued by substandard parts SA use to replace the USGI parts that they no longer have. A good examples are the bolt, trigger group, and op-rod. USGI parts are all milled, SA aftermarket parts are cast.