You can't ignore the increase in BTC value. Say I buy an S3 today for 0.7 BTC (about $350US) and with it mine 0.5 BTC over a couple of months (at current BTC valuation) at which point Bitcoin shoots back up to $1000. I now have $500 worth of BTC in my account, an S3 with a projected estimated value of $150-250, and I am only out $350 and the electric cost.
If you rely on the increase in BTC value for ROI you're better off just buying BTC. This has been discussed to death in various contexts and I've yet to see a compelling argument as to why it would be smart to make and indirect investment in BTC via mining hardware and take on multiple risks (exchange rate, hardware failure, bad luck/variance, difficulty increase etc) as opposed to just buying BTC and having only one risk (exchange rate).
In other words, if I pay BTC for the hardware my full ROI goal is the same amount of BTC. Not USD.
If you still think BTC value will help with S3 ROI look at the exchange rate for the last six months.