Perhaps HPi shells are made in the US so there is no shipping cost (to them).

But about Yokomo shells, I think they're way better quality (in detail as well as material) than HPI shells. Where the HPI shells have only some vague detailing, the Yokomo are (atleast some) exquisite. My experience with HPi is one of the worst ever too, having bought two shells which shattered like glass on their first outing in mild crashes, so I never went back to buy their products.

And in Australia they're just as expensive as anything.