First, I believe you will have to really want to quit smoking and not just want to save money to be successful. I am not sure from your post if you are coming from that perspective but I will assume quiting smoking is the priority.
That said, you will have to be very smart, careful, and lucky I think to save money vs. RYO, at least in the short run. (In the long run you should have no problem saving money when you consider all the costs of smoking for the rest of your life)
You can get in pretty cheap with a 510 battery and some cartomziers and some juice, but you have to consider the battery may crap out fairly quickly if you are not lucky, and you will want at least 1 backup at all times (probably more as the battery life between charges is short) if you are serious about not smoking.
Juice is generally considered the most or one of the most important factors in enjoying vaping. The catch there is that it is expensive to try a bunch of different juices to find the ones you like best. If you have a brick and mortar that lets you try juices before you buy, it is probably cheaper to start there, even if the prices are higher than online.
In the long run, it would be cheaper if you find a reliable battery mod (that allows you to swap out batteries). But this is a treacherous path and you have to buy wisely and resist the urge to try different set ups. If you are lucky to find this without much trial and error and find a juice you are happy with and buy it in large sizes you probably can do it cheaper than you are with RYO.
The problem I and many others have is while I enjoy vaping and don't want to go back to cigarettes, I keep reading this forum and finding out about different mods (the battery part) and different delivery methods (atomizers, cartomizers, tanks, rebuildables, etc.) that I want to try. Throw in the occasional defective cheap mod and cartomizers or juice you bought but don't like. ***Or if you are sure you are going to want to keep vaping, just buy a Provari for under $200 and skip all that jazz, it will save money for you if you are going to be doing this for a good while*****
Probably the cheapest and best tasting deliver method is some sort of atomizer with a drip set up on a basic mechanical battery mod. But this is also the least convenient. If you want your costs to be low you have to watch out for electronic battery mods and watch out for mods with bad power buttons, many that can't be repaired.
In short, in my short time vaping, I have found it can be tough to save money, and I was paying about $5 per pack. $12 per carton is a tough challenge.
The question is - is it worth it to quit smoking? For me yes. What am I willing to pay per year to be smoke free? I would say upward of an additional $1000 over the cig cost, easy. Others would come to different conclusions based on their circumstances. The good news is you can start the cheap route of a couple 510 batteries and test the waters, if you find it gets you off cigs, then re-evaluate from there.
Good luck on quiting. I was a pack a day smoker for 30 years and quit the day I got my first 510 battery and atomizer. Not one since.
