However, at some point you want to revisit one of the first places just to chill there .. and all the enemies are just .. meh. Of course, that's how it is, but in my opinion not how it should be.
You either can scale the numbers of said monsters higher so that they're a threat, or you can replace those monsters with other monsters. You don't much like the former, which really leaves the latter as the only viable choice.
One important question I have with this scenario is this: why is the player there? Why has the player decided to go back to an old area? Are they looking for something? Did your gameplay require them to go there? Do they believe that the gameplay required them to go there (that is, are they unsure where they need to go and are just trying everything)? Or are they just "chilling"?
Well, if the player is just "chilling", then I would say that the player isn't interested in challenge. After all, your gameplay is (presumably) pointing them in the direction of where the challenge actually is. So if they've deliberately decided that they don't want to go there, maybe they don't want challenge.
So maybe you shouldn't force challenge upon them.
If the player is investigating old areas because they think something new may be there, then the presence of new monsters might give the player the impression that there really is something new there. If the game designer went through the effort to put new monsters there, they must be guarding something.
So if you do that, I would say that you have the responsibility as a game designer to actually put something there. Making the player believe that something will be somewhere when it isn't is a pretty large degree of mis-communication. And game design is all about communication.
At the same time, the lack of new monsters would also be indicative of a lack of new content. So if you want to put something in an old area, one gameplay mechanism to indicate this is to add new monsters.
The progress you made at the beginning starts to get smaller and smaller the further you progress through the game. "Oh, you saved 100 gold from the first hour? Sweet, now you get double that amount within 2 minutes slaying this high end monster." Meaning it's not only an issue of increasing numbers, but also of spend effort and time that gets worth less and less.
I think you're measuring "progress" incorrectly here.
When I play such an RPG, "progress" to me reflects what I have. Gold is not really a thing I have. It is useful only to the extent that it can be converted into something that's actually useful. So gold in and of itself is not progress; gold buys progress.
As such, I feel progress when I get a new weapon or some new armor. I feel progress when I'm doing more damage or taking less damage. How much money I spent to get that progress is irrelevant; what matters is ultimately how much progress you make over a certain period of time.
And therefore, absolute flux of gold (gold-per-unit-of-time) is not particularly important. What matters is its flux relative to the actual cost of items. That is, it is the item flux that matters: items per minute.
So long as the cost of items increases proportionately with the flux of gold, then the player's item flux will be constant. Indeed, most RPGs will have lower item-per-minute numbers at the end of the game than at the beginning, despite the fact that monsters may be dropping several orders of magnitude more money than they did before.
So ultimately, what you need to look to is not how much gold in absolute numbers you're giving the player, but the gold relative to the desired level-appropriate items.
If the use of larger numbers truly irritates you though, there are ways to avoid dealing with ever increasing sums of money while still having control over items-per-minute.
One suggestion is to remove money from drops entirely. Make money a reward for doing things in the game, completing quests or whatever, not for killing random things. So it doesn't matter how many things you kill; you don't make money by killing stuff, so you don't have to give more money just for killing bigger stuff. And since you're in strict control over how much money you give out for each quest, you can decide exactly how much to charge for items. And therefore, you can maintain control over item flux.
Another suggestion is to make it so that money drops based on the relative difficulty of the fight. If you're level 10 and the enemy is level 1, decrease the amount of money you get for killing them proportionately.
Of course, this poses a problem: the player may level up faster than they get enough money to get what they want, which could leave them in a sticky spot. They might have to grind out lower-level mobs for crap rewards just to be able to afford the stuff needed to progress.
But in all of these cases, you need to re-evaluate the prices and availability for items. You're giving the player consistent purchasing power throughout the game; you need to make sure that powerful items are either outside of the players' price or are simply unavailable.