RA-Tech NPAS system for SCAR usually go around $40-$50.
I believe this kit (
http://www.airsoftglobal.com/product...ducts_id=11964 ) is worth it for $104, you get all the upgrades right there and won't break down. Basically, the price you pay for Steel Part66 with Steel knocker assembly is about $40-$50 together, you get that upgraded hop up chamber and internals basically for free with the package.