I've found Trek and Tirreno aluminum frames around $80 to $200, some with forks, some without.
Looking on E-Bay, the CAAD 9 frames seem to have asking prices of $300 to $500 or so, with some selling as low as $250. But, more seem to be waiting for buyers than selling.
Anyway, for me it would seem a little high, but in the ballpark as long as it is in good condition, with NO CORROSION.