There are over 100 million long guns in this country. Say one-third are semi-auto, that's 33 million firearms. (Haven't even talked abut handguns.) 33 million times $500 is $16,500,000,000. Yeah, 16-1/2 BILLION dollars.
I think the days of $500 AR rifles are long behind us. You could be looking at $6 Billion just for AR's in private hands at current prices alone. That is the price of a new Air Craft Carrier. If they want to try to offer 2.5X that you are still looking at $15 Billion just for AR's.
Cuomo may think a confiscation would be cheaper but it would cost a lot more than that.