My rule of thumb, as a single practitioner, is that the cost per treatment should be 10% (or less) of the treatment cost. If you are charging a customer $100 for a facial your product cost for that facial should be $10 or less. Of course you have to deduct from your $100 the cost per square foot and other sundries! At this cost point you are able to offer discounts to entise new clients and still make profit.
It's hard for me to say exactly, but I try to keep my backbar costs for a basic facial under $5 if I can. I buy bulk products and use them with great results. I do have a few higher end tx that I do for special clients, and that bumps up the cost to $9-10.
I tracked all my costs and each facial runs me about $6.15. I now use Image Skin Care (I based my costs on using the Sonage product line), so I will probably run the costs again. JM