That helmet is made thru a time & money costing process of sculpting, molding and casting. A lot of clay, silicone and resin is used to make this. If u want cheap try making it out of plaster bandages over a cardboard armature. Then finally painting it. See the Iron Monger Helmet tutorial. Have fun!
I used acrylic spray paints, black primer and silver top coat. Then I go in with black shoe polish and stain it and rub off. Make the silver look aged & bluish + darker. The resin is use is bone white so the black primer is good to keep it looking solid.