You need about ~10m gold to buy the level 12 blacksmith/jeweler/mystic upgrades, I'd say. After that, gold is only needed for two things:
1) Upgrading gems. Not sure how much in total, I'm really just guessing here but IIRC it's about 8m from Marquise to highest tier.
2) Reforging at the Mystic. Now, how often you can reforged is constrained by your available material. Since vendor items aren't salvageable anymore, you are constrained to the blue and yellow items to find, and those aren't too many, to be honest. I personally stop after ~10 reforges and if I don't get the desired affix, I throw the rare item away and try reforging a new one. That way you can get by with relatively little gold (50m or so). If you are stubborn and reforge the same item a 100 times, the gold price will go up to insane levels.
So... the difference between 50m, 500m, and 5b is the luxury of being able to neglect diminishing returns on reforging items. On the other hand, I really believe it's a waste to spend too much money on expensive items on live right now. Mempo? Nope, is gonna be replaced rather soon. Witching Hour? It needs to have RoS stats on it. I tried reforging the WH and all I got was 1 or 2 pick radius (I tried 10+ times!!!). However, even a perfect WH will net a huge EHP loss in RoS. Is it worth it? Depends on the build you're going for in RoS. It's certainly the way to glass cannon. I have a WH with allres but still lose 25% EHP compared to my random rare belt in RoS.
There might be class-specific items you can buy (e.g., wizards want to buy a 10 APoC weapon, preferably high ED Chantodo's). But again, don't go crazy on such items.
TL;DR: If I were you, I'd look into items that *might* be useful in RoS for each of my classes, buy one of those for less than 20m, and keep 200m-300m.
I am saving as much gold as I can so I can reroll stats when RoS comes. I feel I really need gold
This will, IMO, only be very important when you are max level anyway, why reroll stats on gear while you level up? Unless it of course is a item you know you will use for many levels.
What exactly do you mean? I don't understand what you're saying... getting to level 60 now takes 2 hours when done with powerleveling, and the road from 60-70 is pretty quick even if you level up yourself without any gear to help you. As soon as you have one level 70 char you can get level 70 gear with up to -30 level reduced requirement and leveling from 40 to 70 should be a matter of very few hours (if done solo). Every one of your characters will be max level in no time.
Anyways, even at max level infinite money won't help you that much because you still have to find all the materials for crafting and upgrading, and in the process of doing so you'll get gold as well. I'd say 50m gold is the minimum you want to take into RoS, and 500m is the "sane" upper limit, everything beyond that is really not gonna make that much of a difference.
Yeah, I leveled the first few characters on my own as well. I never did powerleveling in D2 either, it was a bit different there as well. But in D3 there's really no point, especially once you know the class... it just feels different and my gut tells me that in D3 powerleveling is okay.
Maybe leveling a new char with the help of bounties is more fun in RoS, but in my opinion forcing yourself to level a char without your friends' help is just an unnecessary self-constraint like playing self-found in D3V.