I don't think there is much difference in price between the cheaper and the higher priced one. I think you just pay more for stronger metal but you also get added feature such as oil or double oil channel.
I am leaning towards this $15:
http://www.airsoftparts.ca/store2/in...roducts_id=967
"Double oil channel design
These are made specifically for the
old version Jing Gong gearboxes, which use thinner and slightly wider 6mm bushings. New version JG gearboxes take regular 6mm bushings."
The older JG g36c had the nylon bushing. I believe the newer ones all went to metal bushing preinstalled. Newer JGg36c is called version 3 with metal bushings.
You can Pm Jugglez (he owns airsoftparts.ca) he probably knows more.
But I just used a $20 set of bushings in mine but I noticed two of them kept falling out if I turend the half shell of the mechbox upside down. But it all fit in the end.
IIRC these ones I got from another seller
http://www.airsoftparts.ca/store2/in...roducts_id=676