I avoid the spreadsheet method like the plague, there are some things I just don't want to know
This.
It's always entertaining seeing those, "I've been vaping for (X) years, saved (y) dollars, and avoided (z) cigs!" banners. 'Cuz I've been vaping for 7 years and if I take an honest look at how much I've spent, I've saved about $0 dollars. It's probable I've spent more vaping then I ever did buying smokes.
And, to give my own answer to the perpetual question here, "are you done stocking up?" seemingly the answer is
no, not ever. It may be more accurate to say I have an excellent foundation/safety net, where I can keep going with my vices for 15+ years, but I'm at something like the 17th "this is my last order" and ya know what, they're not ever gonna stop. I was done by August 8th. But then I had to get Eden Mods Rose v3, it was good, so I needed to get another one because obviously it's urgent, since there will never be anymore 4mL versions made, and I had to get the Lost Vape Therion with the DNA200 chip, because it looks kinda cool, and I got the 75w versions, and... I've actually used neither one, and done nothing but insert batts, see that it seems to work, and packed it back up. And I keep buying ProVari's because they're basically free these days, and I need the Dicodes Dani Extreme V3, because ... well, because it's new, and I need the SX Mini G+ because... its' super-busy SpiderMan Gear design speaks to my inner 5 year old, and then I should probably get another 150 O rings in assorted sizes, eleventy-three more batteries, and 4 more orders from Fasttech (oh hey they have a sale tomorrow I think, yay!). OMG do I have any PC tanks for this obscure atomizer I don't even like laying at the back of a drawer? Probably I should, after all, they crack, and that atty I'm dissing may one day be the very last thing I can vape with, so I should get 25 spare tanks for it, and... while I'm at it, why don't I get another 5 attys at Fasttech 'cuz they're so cheap anyway, and that remind me...
The FDA deeming regs have considerably sped up my spending. I used to go through phases where I'd buy all the cool new shiny things that hit the market, and then lose interest for 9-12 months before repeating the cycle. Now, I've just been buying more and more stuff since the deeming regs hit, and still haven't stopped.
OTOH I did invest lots of money in nicotine, an asset that is going to rapidly increase in value, and I'm all set to start a small additional business as a black-market vape shop attached to an .onion node as soon as the states finish destroying whatever is left of the vaping marketplace within the US in 2017.
The only real savings I've experienced is DIY. I managed to mostly avoid this from 2010-2016, because I convinced myself it'd be difficult and something I wouldn't be good at -- but since loading up for the vapocalypse, mixing some stuff, and playing around with it... I've discovered that while I'm not very good at mixing (and that's being kind), it's pretty easy to follow the recipe's, and then adjust them to suit my own tastes, and arrive at 4-5 things that I really like vaping, which wind up costing me about .5 cents for 30mL, vs. the $20-$30 a bottle I was paying a few months ago. On top of which, I've made the weird discovery that taste is very subjective and mine seem to change rapidly, because when I break out some of the high-end juice I used to buy 6 months ago and try to vape it... much of it tastes a lot like children's cough syrup mixed with glue, "I liked this? Why, it's terrible."
In conclusion: DIY is actually making me pretty happy, and saving me lots of money -- for real, not just in theory, one day, maybe -- go figure. Stocking up on hardware... not so much.