This is why i clearly put it in my assumptions... I know it will change, but not by how much, so for the sake of the calculation, i kept it at the current diff, but i clearly indicated this fact so the OP knew he had to think about this.
So this does change the price of gear a good amount. You are right about difficulty though we don't know if it will be a smooth ride of next gen comes out and just adds ton of has... time will tell.
I didn't think about coupons, good point
You need to add shipping cost and POWER SUPPLIES cost. Diff will jump quite a bit now that Bitmain has lowered the S7 price and all the fools will jump in and buy them.
I did add shipping costs, but forgot about PSU's, thanks for adding this...
All in all, i think my calculation still paints a picture for the OP, it wasn't meanth as a guarantee of income, and i did add a lot of assumptions at the top, so the OP should know this is a guesstimate, and can change if any of the assumptions change