For resin, fiberglass and Bondo, I have the prices in Canadian dollars from Wal-mart.
Resin: $17.22
Fiberglass: $9.95
Bondo: $10.98
I spent (with tax) about $42.00 on all of that, and that is enough to make roughly 1/3 of a costume (if you know how to use it properly-you could end up using way more). Its really not that expencive, and your plastic cement idea will have costed about half that, but would not have worked.
Paper-mache might work if you don't bondo your helmet.
Rondo-glassing your helmet might work aswell (Rondo-50% Bondo+50$ Resin).
Or slush-casting your helmet will work, if you want to spend about $100.00 on a gallon of Smooth-Cast 320.
If you want to have nice armour, you will have to learn this is not a cheap hobby. People spend hundreds, if not thousands of dollars to make the perfect costume. If you don't have money for this kind of thing, maybe right now is the wrong time to get into costuming.