foam is definitely cheaper. i imagine, 100 bucks in foam get you a full suit, and after that you would only need pva glue to seal it, and then a couple of spray paint cans to paint it, so somewhere around the 150 bucks mark.
ive seen amazing builds with foam, but in general, its not as detailed. and nowhere close to as durable. still. for costumes i prefer foam lately. its way faster, with some effort, you get decent detail, its cheaper, and as you really dont wear it that often, it has some lifespan to it still. also, you wont worry so much about it getting damaged
if i were you, id go the route most people are going these days with these im suits. build the suit out of foam, the helmet out of pep. because effectively, its the helmet that gets you the attention.
If you want to go really budget, look up the 25$ iron man build here on the forum
