The electric costs on BlockChain outweigh the profits.

BlockChain.info/stats

They estimate over $1K in losses because of electricity costs. I get what your saying, but its just not good planning. You need to think ahead further than "I buy this and make a lot of money". There is a chance the block takes longer to break than you think. Theres a small chance the thing breaks mid-block. There's also the heat, which has lead to police raids because the cops think the heat in the house is from lamps used in grow houses to keep pot growing. If that happens, you get billed just for the raid.

I know that's not very useful. But, hey, what's the worst that could happen? Besides bankruptcy. Or a SWAT raid.