To work out you actual cost you need to account for everything you use.
So for example you get between 18 and 25 tans per litre (I aim for a minimum of 22 but do my costs based on 18).
So I divide the cost of a litre by 18 (that will give you the solution per tan cost) add to that the cost of 1 hat, pair of sticky feet, a portion of your insurance costs, your petrol, rent, electric, water, telephone bill, washing powder (if you use towels instead of sticky feet), a share of your advertising costs, a share of your barrier cream etc. etc..
You have to think about everything involved in the service and once you have allocated each item a value, you add them all up, that will give you the actual cost to you, from there you can then decide how much you want for your time and you will know the minimum you will want to do a tan or any other service for. This information is important for when you decide to do any offers as you dont want to work for nothing or be loosing money doing any service.