Hey there, you definitely have the right idea for saving money. I started vaping when I had to go on SSDI and just couldn't afford smoking anymore. 4 years later, I'm a huge vape geek.
For a good, cheap mod, try the Eleaf Pico. You may be able to find it cheaper, but here's an idea.
$25.83 Authentic Eleaf iStick Pico 75W TC VW APV Box Mod - 1-75W / 100-315C'(200-600F') / 1*18650 at FastTech - Worldwide Free Shipping
There's tons of options, though. I do recomend that kind of mod with an external battery, because if something goes wrong with the mod, that's all you have to replace. You can still use your batteries on a new one.
Get at least two of either of these batteries, so you can charge one while using the other.
LG HG2 18650 3000mAh 20A Flat Top Battery
Samsung 30Q 18650 3000mAh 15A Flat Top Battery
Depending on how much you vape, a pair should last you 6mo-1yr.
This is a good solid (and cheap!) charger.
Nitecore i2 2 Channel Battery Charger
If you want to start rebuilding your coils, that's where the real savings kick in. There's a small initial investment, but after that you're pretty much set for life.
https://www.imrbatteries.com/nitecore-i2-intellicharger-2014/
These are both great little rebuildable tanks.
$20.64 Authentic UD Goblin Mini RTA Rebuildable Tank Atomizer - 3ml / stainless steel + glass / 22mm dia. at FastTech - Worldwide Free Shipping
https://www.fasttech.com/products/3028/10018935/6654400-authentic-wotofo-serpent-alto-rta-rebuildabl
I can walk you through what wires to get, tools, point out some youtube howto videos if you're interested.
If you aren't mixing your own liquid, you should start. You can spend pennies per ml instead of dollars. Again, that's an initial investment, but very much worth it.
Hope this helped, good luck, and feel free to ask me or the forum anything. People are great here!