Not to much tbh.
Look at ti this way if you are a heavy smoker , it cost's a fortune to buy decent amounts of good bud , if your lucky enough to find it.
I can easily go threw 1-2 ounce a month without trying. That would cost me approx £300-£400 If I was to buy.
So the way I see it I can spend up to £200-£300 a month on my grow and it's still worth while.