i was calculating based on my own experience as well.
one bag of fox farm for $14 (i can buy for as little as $12.50, but need to spend $2.50 on gas to get it, so the $14 bag is cheaper in the end) can fill 3 of my buckets. i do 6 buckets, so 2 bags per cycle = $28. add perlite, which is $18 for a huge bag that mixes 6 buckets. so $28 + $18 = $46 for one go around that does not pull a pound a month, as your original calculation said. if you want to pull a pound each month, double that.
don't forget the gas money required to get you to the store and back. and i believe that gas goes into a car, which costs money to buy, insure, and maintain.
don't forget that the plants take up a certain percentage of the space of the home, which costs money to rent or buy.
and don't forget that tents are not free and do depreciate, just like lamps, fans, and everything else (a good accountant always factors in depreciation).
and lights don't hang magically from the sky, you need hardware to do so. and you need to go to a store to get it. which costs money as well.
do you grow indoors in the summer? be sure to account for the air conditioning, which takes up many kilowatts.
do you water your plants with free water, or does it come from the tap? do you pay a monthly water bill and use a certain percentage of it on your plants? factor that in.
ever get bugs, mold, etc? factor in the cost to control those issues.
in any case, the calculation i come to is generally in the $105- $120 per ounce range for top shelf, indoor meds.
that is giving away my labor for free, which is in the range of 20-25 hours per week.