As far as cost increasing you might have to buy more fiberglass mat/cloth but the resin you should still have plenty of. 9sqft of fiberglass mat is like $7 local to me. Don't know what the price is in your region.
I can't speak for that wallpaper paste method of paper mache since I have never done it, but I'm confident fiberglass would be more durable by far. I mean fiberglass is used to make boat hulls and bathtubs.