So if difficulty goes x20 then you are at 200 days for a D3 payback assuming the same price for Dash. If it's x40 then 400 days. I think x40 is the outside amount and it will be less than that. So yeah D3 isn't a get rich quick opportunity unless Dash pricing goes up a lot. Speaking of this, interest is definitely increasing and having all these mining manufacturers drawing interest can only help so I would expect Dash price to start tracking up over the next six months.
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?
I agree and despite the trend in this thread difficulty and price are correlated to some degree. It's not like the 2 factors are detached and it doesn't take a genius to see this. Impossible to predict the outcome but as a few guys said after the L3 shipped they were upset they didn't buy more once they got them up and hashing.
Rjenkin is right about the passing of the torch from the barely profitable x11 farms to the new Asics. I agree that 100% increase in difficulty could be offset by those farms switching off Dash. We can all only speculate and we also don't know if Bitmain intends to mine dash or is just testing the Asics before shipping then closing the chapter on them. Current spike in difficulty could be attributed to bitmain mining and then diff decreases when they ship. Again this info isn't being made public but I do wish it was so we could all better estimate.