right now (doing the lengthy math) the cost of mining is a multiple of 104 of the amount of exahash mining.
so 32exa * 104 =3328
this 104 is based from maths of a s9 sold at $390, that is 1.3kwh with $0.05perkwh cost and is 14thash
if you want a easy multiple of the new s15.. that would be 117. but they are not out for delivery yet(deliveries begin in about a fortnight)

if the entire network was running s15 then the cost of mining at this weeks 32exa low, would be $3744

mining older hardware which you have already broken even in regards to hardware where you are now paying just electric
is 61.9x of exahash (based on $0.05/kwh)
meaning if you bought an old asic in late 2017 and got your $2k rewards after 5 months(i done that math too) and now just mining with electric as your cost
32*61.9=$1980.80

so s9 mining =104x of exahash
so s15 mining =117x of exahash
so electric only cost mining = 61.9x of exahash
use those numbers and you will have a quick and easy way to work out which is cheaper.. to buy or to mine

have a nice day

the reason i am using 104 as base cost. is that it is the cheapest hardware+electric majority.
those with earlier rigs that paid themselves off, most people use the hardware payoff to then buy new rigs so technically its not just pure electric as their hardware profit is respend and gone. so they are still in the loop of trying to recoup