I believe the number assumes that you can get hashpower at the current price for (a small amount of) hashpower. However, as demand increases, the price goes up,

I believe the number assumes that you can get hashpower at the current price for (a small amount of) hashpower. However, as demand increases, the price goes up, making the attack more expensive in practice. Also, if people notice what you're doing, they'll probably stop selling hashpower to you (since you're attacking what makes their hardware valuable), driving your costs up further.