Once you get all your gear purchased (and seldom has to be replaced) the only cost you have to incur is for your juice and for coils. Even at $2 per coil, lets say $8 a month for those, and $10 for juice, you're still at less than or close to $20 per month. Anything in addition to that is called a hobby, and not essential.
Well I suppose if you plug in with a cord all the time like I would love to, then the only recurring expenditure could end up being liquid and coils.
In practice, people are going to be changing batteries.
How many batteries per year: well I suppose it's possible for somebody to get by purchasing 1 battery per year; then again, for some people it might be more like 10 batteries per year if they are using the small ego batteries.
I'd guesstimate something like $50-$200 per year on batteries; monthly that works out to something like: $5-$20 per month.
Coils: $2 per coil and 1 coil a week; 1 coil week is pretty optimistic; I can see some people running more like 20 coils per month until the figure their way out; on the other hand 1 carto a week sounds like what we hear from the carto users;
I'm going to budget the figure at $20 per month on coils/cartos
eliquid: more like $3 a day - works out to about $100 per month
Actually it's worth considering that at startup, there's the initial waste on liquids that the user doesn't like, then there's the stage where they find a liquid they can use in place of cigs; after that, over time, there's the gradual progression towards chain vaping, then double that when they start vaping on dual coils.
Meanwhile the startup cost just getting started with gear and looking for suitable replacement of cigarettes ends up costing anywhere from $100-$1000 during the first year.