I've tracked every single purchase I've made for vaping.
Before I started vaping, I was smoking over 3 packs a day. 2 cartons a week didn't quite keep me in cigs. I was smoking the cheapest cigs I could find, at about $40 a carton. That's $80 a week, $320 a month.
I've been off the cigs for six months, and had cut down on my cigs quite a bit for a month or two before that. (It took me some time to completely switch - turns out I needed a higher nicotine level before I could drop the cigs for good.)
Six months, * $320 a month is $1,920 that I would have spent on cigs.
What I've spent on vaping is $1,323.28.
That includes the eGo's I started with, a Buzz Pro, a lavatube, a cheapy VV box mod, liquinator tanks, cartomizers, juice, and misc stuff. It includes a tool box that I keep some of my vape-stuff in, and a meter, various battery chargers, extra batteries, a little tiny screwdriver set, a tool to punch cartomizers, all kinds of stuff. So for me, I'm easily $600 ahead after six months. And my savings will start to add up a lot faster, because now that I have everything, and pretty much know what juice I like, I won't be doing so much experimenting, I won't be buying as many mods and tanks, and I have tools, bottles, etc.
Now that I have both knowledge and a good collection of basics, I'm pretty much just buying cartomizers and juice. I vape a lot, but $100 a month from here out will cover almost everything. Eventually, I'll probably want a 2nd lavatube or possibly a provari, but since I already have equipment and backups, I'm in no big rush.
The less you smoked, the harder it is to hit the break even point. Someone who smoked a half pack a day takes longer to recover the getting started costs of vaping, for instance, than me. But they will also likely use less juice, fewer cartomizers/atomizers, etc. In time, they'll come out ahead, too. It just may take a bit longer.
To me, the cost is the secondary reason to switch. Health is more important.