Add up ALL of the initial costs, and divide each by the number of months (or grows, I suppose) they will last before you need to replace them. If you grow with one 600-watt HPS and get two grows out of a bulb, your expense for "bulbs per grow" would be half the cost of the bulb, per grow. Tents will last considerably longer, making their cost per month and/or grow considerably less in terms of percentage. Nutrient costs would be calculated the same way. A pH pen might last a long time. Its probe will last less, but will still last quite a while if taken care of (making those calibration / storage substances a beneficial expense). Et cetera. If your grow space is a room/cabinet/structure instead of a tent, it should last many years - but one still has maintenance costs, such as paint/primer (or at least cleaning supplies - count those, too!).
I guess spreadsheets really do have their uses :rolleyes3 ... But a person could do this on paper, too.
AfaIK, this kind of thing is what people in "the trades" (plumbers, landscapers, roofers, et cetera) who own their own business - and, therefore, are responsible for purchasing and replacing tools, consumable materials, etc. do in order to not end up losing their shirt over "all the little things."
When figuring up electricity costs, add EVERYTHING that is related to your grow. That means that if you have central air conditioning and it is running more often due to the heat your grow produces, you need to count that extra bit, too, lol. You can look at a previous year's electricity bill (for a given month) to help guestimate, I suppose, but it is better to find out for sure. If you added a window a/c for the grow, count that device's electrical usage (and count the a/c, itself, as part of your ongoing materials cost!). Fans? Electricity. Pumps? Ditto. Add it all up. For anything that plugs into a 120VAC electrical outlet, you can purchase a cheap "Kill A Watt" meter that plugs in and has its own outlet for you to plug whatever you wish to into. This device will show you how many watts (et cetera) the thing that you plugged into it uses. It'll also display in kWh if you like.
People have suggested, rather than grams per watt, grams per kWh might be used.
