Target sights on that model carry a 2 to 3 times price premium if factory. Going higher than $500 would not be out of the realm of a fair price depending on what the gun looks like. One gun with 65% finish is not always the same as another gun with the same amount. It depends on whether there are dents or rust and abuse, etc.
I have the same question as Oyeboteb above. Also is it a pre war sight?
If so, an easy way to verify if the sights are factory original is to remove the rear sight and see if it's numbered matching to the gun on the underside.