Nordic tubes are metal too. Aluminum is a metal. TacStar is carbon-fiber and a lot more money at $65 from Cabela's.
The Choate from Midway is a bit less expensive at a max of $45.99. Mind you, you should shop around. Prices vary but only by about $5 or so. Brownell's, for example, wants $41.99 for a 7 round Choate kit. $49.99 for a Remington brand 4 rounder. $45.99 for a 10 round Remington brand. All in 20 gauge.