The KDG MREX only adds 3.7 oz to a SCAR. Very, very impressive. The extended top rail is not receiver height. They show it off with lasers or Surefire X400Us mounted on it. While not new, the PWS SRX extended rail has a lower top rail similar to the MREX. The MREX however is a one solid piece design and weighs less than 1/2 of the PWS SRX rail!
The MREX retails for $299.99
Midwest Industries has updated their SSR rail with Keymod holes. It weighs 7 ounces. A bit cheaper at $239.99 The SSR does have a receiver height top rail but you lose the use of the factory front sight.
Handl Defense is coming out with their own SCAR rail and it is made of Carbon Fiber. It is the most expensive of the new offerings at a price of $500 for Black Carbon Fiber or $530 for Tan Carbon Fiber. Surprisingly heavier than the MREX at 5.8oz.