I'm not sure when these became so cheap, but Primary Arms has them on sale for $3.99. I seem to remember these being close to $50 a while back:
https://www.primaryarms.com/Daniel_Defense_Vertical_Foregrip_Black_p/21-067-19000-006.htm
https://www.primaryarms.com/Daniel_Defense_Vertical_Foregrip_Black_p/21-067-19000-006.htm