If Dash "goes up a lot" you're likely better off just buying Dash. In other words, I think you should calculate the ROI/payback/whatchamacallit in Dash, not USD, or rubles, or whatever. Will the D3 mine more Dash than the cost of the miner in Dash at the time of purchase?
For batch 3 the number is approximately 7.5. Will it mine 7.5 DASH or more?
That was my exact thinking when I finally decided not to buy one after having spent a couple hours slowly progressing through the website, hitting refresh over and over, going back to start to get one in the cart again. On the final page I finally bailed and went to sleep instead. The fact that there were lots of people in this thread saying they had managed to make multiple purchases and that it had been two hours since it initially released also had me suspecting that the batch size was actually quite large making individual profitability that much lower when these finally do ship. I'm sure batch 1 buyers will make a profit, and possibly batch 2 buyers as well, but I am going to wait for the next generation of more efficient miners when the network hashrate will be a lot higher and much less affected by a single model which will also make profitability calculations that much easier.