OK, super quick as I only have a few more minutes...here is an example of what a cheapish gas gen-set and a lot of gas would cost for one growing cycle of flower only. Why flower only? For simpleness of the math. We'll assume you preveg in a room run off the house power. We'll also assume room set up is over and above these figures.

I take a quick online peak and lets go with a Honda engine at least. These 3600rpm motors are not supposed to go for the long haul, but we should get one crop out of them? Now...lets say you do in fact only get one crop out of the gen-set and then just toss it. 1500.
That unit is a 6875W peak with a rated 5500W continuous. No matter what gen-set is used for continuous duty, an 80% load is considered max. But Honda gives us a fuel consumption usage for 50% load....so lets fire up 3 1K's and do the math. We'll be close enough for our purpose here.

.5gal of gasoline per hr running our 3000W.
.5 x 12hrs = 6gal per day
6gal x 56 days for an 8wk cycle = 336gal
336 x 3.78 = 1270ltrs
An average estimate of 1/ltr for regular gas = 1270

1270 in fuel expenses
250ish in oil for oil changes
1500 for the Gen

3020 total expenses for 56days

3000W = 3 bags for sure, 3.75 average no matter what, 4.5 if you're doing good and 6 if you're expert.

The super markets will pay 2000 for a nice big bag of peppers. Have you been into a Safeway recently? It's like 3 ea for a nice Bell Pepper.

So lets do some hypothetical math.
2000 x 3(min) = 6000 - 3020 = 2980
2000 x 6(max) = 12000 - 3020 = 8980

Is that^ by itself worth the hassle? Your call.
So lets figure that you already have the gen-set running and all is good. So why not run it 24 hrs? It runs 3K at once...which could be a 3 and 3 flip-flop. 6K total. Lets do the math.

Double the fuel - 2540
Double the oil - 500
Double the gen-sets - 3000

6040 total expenses for 56days

2000 x 6(min) = 12000 - 6040 = 5960
2000 x 12(max) = 24000 - 6040 = 17960