I've seen them down in the $350 range for a dedicated flat-top upper with one magazine. I think those are made by Oly. I know that there is usually one in the Sportsmans Guide catalog I get. I have not seen one posted for sale on the boards, but I have seen them in other catalogs.

There are a bunch of discount coupons for Sportsmans Guide posted on the Guns Net boards.

I'd like to have one too. I just have been distracted by other projects lately.