can payment to Bitmain be made from your exchange wallet if you keep your coins on the exchange?

i would like to pay for the D3 miner with litecoin if i can manage to secure one on the 10th when they release the batch

I'm a bit worried that the Roi will take very long  for the November batch , the difficulty is currently just over 600000, by what i've read above some are predicting that it might rise to 6 million which will give about $20 p/d.

Is it still worth buying one of these?