Ok, so I make a model, and upload it. Price is say, $300. OK, fine, I get it. Not a problem. I then realize that I could hollow bits of this piece out, so i do. I hollow it out to the point of reducing the model volume by about 30%. I upload that one to shapeways and it tells me the cost is $301. How exactly does THAT happen? If they're pricing by amount of material used, shouldn't I be at about $210?