Last time I calculated it (about two years ago), to just vape it cost me $6.38 a month. This is the cost of the nic concentrate, PG/VG base, flavorings, wire & cotton used in a 30 day time period averaged over a 6 month period (yes I tracked it - bet with the wife which we agreed to disagree on who won).
I should go through the exercise again as at the time I did it I only factored in the cost of the 2 litter bottles of Nic I had, since I now have a lot more that I bought, and the places I was buying my other supplies have changed, the number will probably change a bit too but not by much.
FYI on the bet I mentioned with the wife, after a "spirited" discussion I bet her that it cost me less then $10 a month to vape. The terms of the bet were if I won, she bought me a new mod, if she won, I bought her a new pair of shoes (not really fair as her shoes were a lot more then the mod). So I tracked everything and presented my number of $6.38 and declared victory! She then went in and added in all of the Mod/RDA purchases (I had a bit of the Shinyitis at the time) and declared she had won.
I bought her, the shoes and myself the mod and left it at that.............