@Olleus
The problem you describe here is referred to scientifically as Cumulative Advantage/Disadvantage and is an actual phenomenon in reality. Cumulative Advantage or Snowballing as we like to call it due to Conquering other Civilizations is indeed a a very severe problem in Civ 6.
However, I think the solution of increasing costs exponentially in response to increased production rates is not feasible and here's why, based on the principles of game design. In every game, there are clear rewards and punishments; without which the game will not be fun. Rewards must always be the result of right decisions and Punishments are always the results of wrong decisions. At no point of time should a player be punished for making sensible decisions and the opposite is true as well.
In Civ 6, rewards come in several forms but most of them trace back to the same root, an increase in relative "power" to complete objectives. One of these rewards is an increase in generation of resources; be it production, food, gold, culture, faith or science.
If production/gold costs are unfairly increased for a player just because he's winning and needs come challenge then you would have broken 3 fundamental rules of game design; all at the same time.
1: Giving a winning player a handicap is simply unfair. Games must be fair, that is why there is balancing in games in the first place. You don't start crippling leading athletes just because they'e taking a significant lead that's just absurd.
2: Giving a winning player a handicap for being efficient in one aspect is directly punishing a player for doing things right when you'r supposed to reward them instead. Terrible.
3: Raising costs for a player who is generating more resources destroys the reward of making right decisions in the first place. You finally start generating enough production to build districts/wonders quickly (Reward) by investing in Industrial Districts/Buildings (Right Decisions) and the game directly responds by raising production costs so that you don't actually get to build them much faster. Where is the fun/reward in that?
You do not cripple player power or remove existing solutions as some sort of challenge, that's just very lazy and very poor design.
The snowballing effect is only exceptionally prevalent because of how insanely profitable warmongering can be. No amount of building and expansion can beat the efficiency of conquering opponent cities in the same time frame.
A better solution to this issue is to
a: Bring back the penalties of conquering cities so that it is not superior to normal expansion. This is only fair since you are already gaining a lot from conquering opponents cities without significant costs. (Units are far cheaper to produce now thanks to the 50% boosts from military policy cards while districts costs inflate to huge amounts.)
b: Reintroduce penalties for having too many cities. Founding new cities should still give significant advantages but there must be appropriate costs and decision making processes to manage huge empires and that's only fair based on reality.