actually i dont think those are too expensive but to be honest i dont really think it would be worth it. I mean those first person shots do look cool but they get boring if you have a lot of them in a video because you cant see very much.
if you dont already have a regular video camera i would get one of those first. because you can film a lot wider variety of angles with a hand held one. and you can always hold one of those cameras and go off a jump with it to get the same effect
if yo u do already have a normal video camera id say go for the helmet cam and you can just bring both cameras to the hill