When you don't have a lot of money...
You make costumes with whatever you have. Flour-and-water papier mache handmade masks figure prominently in this. So do sheets and towels with holes cut in them. None of these are going to look elegant.
When I was a kid, old sheet "ghost" costumes were a favorite at my house, along with old-clothes "hobo" costumes touched up with Mom's sewing skills.
I don't recall ever buying a costume at a store.