You forgot to include increases in difficulty. If BTC/$ increases along with diff it'll pay for itself in those 100 days. Considering what's going on with the price now, illustrating the volativity, I'd say that's unlikely. I'm confident BTC will continue to increase in value, which is why I'm still GPU mining, but you need to consider all factors and make sure you have the money and stomach for the long game.

Remember that the price of Block Erupters just dropped by 50% and once other ASICs are shipping in bulk they'll have to do that at least once more if not two or three times to stay competitive.

Don't jump in without doing your homework.