You start to break even and then make money when you stop buying hardware, except for cartos, atty replacements and the occasional battery replacement. Juice is the cheapest element in the equation.
Even though, I was smoking 2 PAD. At $30 a carton for the cheapest name brand, that was $180 a month, so I had a lot of room to play. The first month, I bought a Joye 510 starter kit. a few weeks later, I bought a Joye Ego starter kit. 6 months later, I bought a couple of extra Ego batteries and the Mega Type A cone atty and then the MEGA Type B atty, followed by the Tank versions of those when they came out. In between my starting point and the Tanks, I just used the standard 510 atty. I bought a 306 atty and a drip tip last and found that to be a great vape for sitting around the house.
Now, I'm into the Boge LR cartos and the CE2 clearomizers. I really like the Boge cartos better than anything I've tried. The CE2 clearos are good, but not quite up to the taste level of the Boge. I still use a 306 drip tip and a Little Dwarf drip atty from time to time. The Little Dwarf is an 801 thread and requires a $4 adapter to fit the Ego. It's a 3 ohm atty and works best with a 3.7v batt and I'm told it rocks with a 5v box mod.
The Dwarf 801 Dripper's Atomizer
I still think I've saved money in the last year and a half. Still smoking, 18 months x $180 would be $3240. I don't think I've come any where near that with my hardware and $50 a month in juices.