lets do some usa math for usa buyer.

buy one at 840 +110 = 950 x 1.276 = 1212-65 = about 1150

high power = 23000gh and 1600 watts  I rounded

2 used s9i on eBay = 500
2 used psus =            170

total 670     vs 1150

hash  23000   watts 1600
hash  27000   watts  2550

pretend they come the same day I am spending 950 watts extra with the s9i  that is 25kwatts a day round up

750 kwatts a month  at 6 cent power that is 45 dollars a month  so in ten months you save 450 in power with the new gear

but you lose 4th a day in hash that is 24 dollars a month  current prices so you don't save 45 a month you save 21 a month for the new gear and you spent an extra 480 to do so.

so buying the t15 at 840 with a 65 dollar coupon is moron move for a usa buyer when he can buy 2 s9i's used on eBay easy peasy.

Bitmain has fucked the game up hard.

now that is a 6 cent power  set of fact  480/21 = 23 months to be even

at 7 cent it is faster  you gain 28.50 a month over the s9i.