It's all a numbers game, current 1 th/s miners will return close to 6BTC. At $1000, that would mean the miner would cost 3BTC ($3,000) and return 6BTC for a 3BTC profit. But at $400, the miner costs 7.5BTC and would return 6BTC for a 1.5BTC loss. You're better off buying and holding.