That will be 30-40 bucks for the Hasbro and then you have to figure in the other materials, there will be sand paper, bondo, sintra/plastic, and visor......etc. I would expect you will be in for about 100 bucks by the time you get done with the modifications. A lot of people will say that they got a Hasbro and modded it and only spent X on it........but most of the time they leave out the fact they spent many many hours and a bunch of other money on misc supplies.
You could buy a helmet kit in the 125-145 range and start off with a much better base. Of course you will still spend money on paint, and sand paper but in the long run I would imagine you would be happier.
just my 2 cents.