My understanding is that Springfield no longer warranties them and when they did, several years ago, they generally offered discounts on the M1A since they do not have any parts for the SAR8.
Prices vary with the Greece ones being worth the most, the stamped steel non-Greece ones less than that, and then the ones with the aluminum receivers being the least desireable and cheapest. As for current price, your guess is as good as anyone's.
I have an aluminum receiver SAR8 and have no problems doing head shots all day long at 100 yards with a B-27 target using iron sights. The gun destroys brass by denting the heck out of the case upon ejection, though.