I’ve seen them here on the EE for $450 at the low end and $550 on the high end. You got it for the lower end of average. They are hard to find though so if you’re actively looking for one it’s not a bad price.
The easiest way to figure the price is to look at the going rate on a full gun and the going rate on the uppers. The difference is the expected price of a lower, about $500 these days.