I've been wondering the same thing, though my main costs have been juice (Tasty Vapor--I'm going to do doublers next time, I swear), and more batteries/attys/PCC's. Then today I got a PT, a USB charger, and another mega battery, a few more attys (I'm scared of running out), and another juice. I'm pretty sure I've hit the mortgage payment--or close to it--but ours is relatively low, and I keep reminding myself that at least I'm not smoking, and that the cost will even out and I'll start saving money...eventually.
I'm going to try and look on the bright side...I want my husband to quit dipping Skoal because I'm scared he's going to get jaw/mouth cancer, and that his teeth are going to start falling out or something, so now that I have enough equipment for both of us to have PV's at all times, MAYBE he'll start using one too. (One can dream, can't they?)