Of course, there are guns that don't have a metal body -G36, P90, AUG, etc. But don't trick yourself into thinking that "plastic is plastic so the brand doesn't matter." The body on the CA G36 is far superior to any other brand I've examined, in both finish and how solid it is.
To echo what CJay said -there's a reason cheaper guns are cheaper. You compromise somewhere. TM puts their higher cost in quality control, Classic Army puts theirs in external quality. I haven't seen enough VFC, G&P, or full metal ICS guns to really develop an opinion on them.
From what I've seen of the brands that sell for under $500, the only one I'd actually spend money on is the JG P90. IMO the halfbreeds aren't worth the cost.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."