Yes it would be a much "hotter" deal if you could use regular air, to avoid the helium charges...but I cant see how that would be done reasonably with so much weight (ie if it could be done with regular air, they would have done it, theyre not makign any money by requiring helium fillups)
...either way, its clear there is a way to make this thing for pennies compared to what they are charging...the expensive part would be the helium fillups (unless you find a cool person workign at party city who hooks you up!!)