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.
The only caveat to that would be something like the S1 antminers, purchasable with btc that for awhile kept dropping in price to keep them an attractive option.
It removes (some) of the uncertainty around pricing if a miner can return more btc than it costs to buy (with some guesswork dif increase %) in a respectable amount of time before energy costs eat the value.
My uneducated guess of 20% increase, at the price I grabbed the miners, including PSU (x6) will, maybe, ROI in 87 days, year end btc profit of 39%
But!
Power supply=free
network connection=free
misc supplies=free
Only down side is the 100A service in the apartment. Been chatting up neighbors hoping they would let me mine for a share of profits.