Full metal bodies cost a lot because retailers can't sell them in the open. The legality is in a fuzzy area. This may or may not change in the future. Given the increased risk and decreased supply of these things, prices are quite high.
I wouldn't bother upgrading that gun to a metal body. It'll cost as much or more than you paid for the gun, and prices of actual full metal guns are set to drop this year as it is.
Game it for a bit, when you get tired of it, pass it on to a new team mate, and move on to a (new) full metal AEG.
Now you know