I did the math on this a while back. I went from $200+ per month on analogs to ~$43 per month on vaping. This is a "minimum", like, if I just did the least I could do & not explore new options, try new stuff, etc., .... just "this is what works for me, and if I *only* do this, what does it cost?" level.
A 30ml bottle of DIY juice at 25mg costs me about $7 to make. (probably less than that now, as I'm buying nicotine, which is the most expensive component, in 100mg / 500ml volumes, but I'll stick with the formula). I'll go through 3 of these (3ml per day, so 90ml per month). $21
In 4 weeks, if I stretch it, I can get by on one 5-pack of 2.0 ohm cartos. $9
I use box mods, or very sturdy other mods (like the P10)... but batteries eventually wear out. A two-pack of batts from dealextreme.com is around $6 for the sizes I use, so: $3 battery allowance.
Boxmod parts are super-duper cheap: I can make one for under $10. $12 if I use an LED. Parts allowance per month: $5
Then I figure in a "fudge" factor of five bucks. Covers shipping and incidentals like replacing lost drip tips... another $5.
Of course, I actually spend much more than that, since I just have to try a few of the new things, or get a new kit, or back-stock on stuff... but honestly, I haven't spent a penny on vaping in almost 2 months now; I've been living on my closet stockpile, saving cash for when my sister comes to visit next month. I could go without spending another penny for at least 6 months... but I always stockpile.
Anyway, you can see that my minimal cost is $43 per month. Let's call it per "4 week period". That would be $10.75 per week, at a minimum. More if you want to make it a 'hobby' and explore.
Beats the hell out of 50+ bucks a week for a carton, don't it?