People have mentioned the word foamies...

In fact, they can be about the most economical models of all. There are lots of plans available for constructing your model from sheet foam. A lot of them fly great and cost almost nothing.

One that's popular at one club field around here is the Polaris. http://www.rcgroups.com/forums/showthread.php?t=922465

This is one that doesn't have to be made from EPP foam to be almost indestructible. There are others perfect as trainers. Although the Polaris isn't a bad trainer itself.