If you were to use foam, pink styro- your cost would approach buying them. I priced 2" and it was almost $20.00/ sheet. Glueing the together is a problem as well. The upside is that regular spray paint eats away at it. if you painted feather detail, wing tuck, tail cleavage, etc with latex paint then spray it the solvent eats away leaving high and low spots. The amount of erosion is dependent on how thick you spray. After it erodes and dries off, it stops and leaves a relatively hard shell. I tried making heads and they looked great! The chips revealed pink!
Just my experiences.