It actually reach $650; in any case it should at least be double to what the miners where earning before. And yea it shouldnt be much of concern if the pay rate is still the same; but if it goes to around $900 I bet that actually compensates for the halving and the mining difficulty.
weren't miners earning on $450 before the halving? therefore it need to be $900 for them to be sustainable, but they also have a better efficiency now with antmier s9, so maybe 600-700ish is enough for them
... except that you can't ignore difficulty, or even assume that it will be constant.