Just the upper for the HK 416 are running upwards of $4000. Supposedly, according to sources at HKPRO.COM, HK will be releasing the 416 type uppers to civvies in the near future, should be around $2k. The Ares kit is nice but you are very limited as far as handguards. I believe they are supposed to come out with a railed handguard for their conversion (might take as long as the SHRIKE!).
I like the LWRC (Leitner-Wise) setup (I believe it is the SRT). Gas piston, same heat reduction/dissapation and they offer it in 5.56 and 6.8. Machowitz (sp?) just did a feature on it on Futureweapons. Showed him pulling it out of water and firing away with no problems (it has drain holes to allow the water to quickly evacuate and not foul the gas system or barrel.