They are pretty desirable, they often go for $600+ here in the classifieds, but locally may be much lower and on the auction sites may be much higher. They are pretty cool guns.
I have one of the 4" 22 Sp-101's & while I don't have the box,, I wouldn't sell it for $500. But the true value is in what you can actually get for it. Check Gunbroker for completed sales to get an idea on value.