With the pricepoint of Prime where it is now, some people will sign up for it and not utilize it enough to break even. Raise it from $79 and you won't have as many casual shoppers agreeing to go for it. You'll be left with just the frequent shoppers. It's called the point of diminishing returns. Raise the price enough and you thin the herd resulting in less profit. It's like the Obamacare thing. If you only have old people who use their insurance a lot and no young people who don't use it, then it doesn't work.