Here is an AR with the lower made from plastic.
http://www.youtube.com/watch?v=8DUmwZ_Sd2s

This guy has experimented with wood as well with bad results but has been successful in getting them to fire. There is a thread about it on the weaponeer.net board... That said, if I had a milling machine and wanted to build an ar-lower I could get a forging (25 bucks) and some tools and whip up something to be proud of... I wonder if this could be built with some feeler gauges a drill and trigger set... No clue as I never saw one. 300 buxs + is what I think these airsofts go for. Add 600+ for a decent upper. Make take down and cleaning an even bigger pain in the ass... There would be no friggen point. I'm (having not actually seen one of these airsoft lowers) thinking it would not be overly difficult. Not even sure if it would be more or less than an 80%...