i would think it'd be only about $100 to make a decent set of molds for a jet pack
OOMOO30 silicone rubber and some fiberglass for the mother mold
about $85 worth of rubbed and $15 for fiberglass resin and matting.
it doesn't need to be thick, just well supported.
of course. if he's doing a run, more money should be spent for a very sturdy mold.