If you are going to build one, here is a good starting point. Aero makes lowers for a lot of vendors, including Spikes. That one in the OP came from Aero. $125 for a lower receiver and DPMS lower parts kit is a good deal. I'm tempted to get one just for the hell of it and build it in case there's another rush.
https://surplusammo.com/index.php?main_page=product_info&cPath=33&products_id=502&zenid=2830e217e0d215ad2ca861c5a8f4ecd3