I'll jump in from a noobs point of view.
I also take into consideration the potential value of the miner after I'm finished with it. I have been amased by how much I got for my antminer U1's that I have just sold. I had 10 of them, each cost £20 when purchased. Each sold for between £12-£14 in the last few days. Once I added that to my ROI calc I was up a fair bit with BTC earned and return on my cash down payment. Now all of my cash is back I now calculate just in BTC and assume that I will get 50% of my original purchase price back from any equipment.
I'll just have to hope I didn't sell to any scammers on E bay
This may not be correct way to look at it but it helps me to justify(in my head) the expenditure.
I also take into consideration the potential value of the miner after I'm finished with it. I have been amased by how much I got for my antminer U1's that I have just sold. I had 10 of them, each cost £20 when purchased. Each sold for between £12-£14 in the last few days. Once I added that to my ROI calc I was up a fair bit with BTC earned and return on my cash down payment. Now all of my cash is back I now calculate just in BTC and assume that I will get 50% of my original purchase price back from any equipment.
I'll just have to hope I didn't sell to any scammers on E bay
This may not be correct way to look at it but it helps me to justify(in my head) the expenditure.