I actually managed to spend somewhere between 300 and 500 million, the price of the potion was not an issue (well, not entirely as my sight got blurry and I had a splitting head ache from having my eyes glued on to the screen and the annoying rythm of clicking) if it was anything more expensive I'd just end up spending more, but so would everyone else. It's just about how long you can endure to actually keep buying potions. I managed 7-8 hours of it (almost crying by 9 pm), and if I'm not the winner (or loser? in life?) I'll be shocked!
Also, the stack thing was a bit of a confusion because of what some people were saying and also that you could consider 10 potions bought at one time as one purchase.
Anyway, good luck guys.