Are you talking about foam core board?
because i bought enugh to do an entire helmet for like 15 bucks at hobby lobby,
or do you want like actual carvable, sandable foam?
because i saw that at hobby lobby like 3 ft by 2 ft 2 in. thick for like 7 bucks a sheet...
that would prolly come out to about 45 dollars
depending on how big your nogin is...
but it may not be good if you want to fiberglass it,
depending on what kind of resin you use because
some resins eat away that kind of foam...
depends on what you want it for i guese