I don't have enough reputation points to leave comments. So it forces me to answer this way.
First of all excuse me if my English is not very good. It is not my native language.
If I am allowed any constructive criticism, generating a price hike for each item you buy sooner or later will be a negative point for the player. I would say that sooner rather than later and I end up producing a displeasure and that in the end I end up leaving the game.
Let's say you buy the first bottle of miracle potion at 5, the 2nd at 55, the 3rd at 70, the 4th at 93, you will be paying the 5th for more than 100.
And from what I'm understanding, you want the boost value to be given regardless of whether you make all those purchases in the same transaction. That is, it does not matter if you buy the 5 together, or at different times.
If you wanted to emulate the market, that is not the way.
If I go to the market today and buy 5 soda cans of the same thing, I pay it at the same price for each can, unless I find a 3x2 offer to give an employment. Tomorrow is something else, and the price may be different.
You should objectively analyze what you intend to do. Define the limits or scope of the "simulation of an inflationary process". That by the way, inflation is not given in that way.
The inflation process is largely due to an excessive increase in the money supply. More info in Money Supply.
But it is possible that achieving a mathematical model that allows you to carry a macroeconomic balance in your game is more complex than you are looking for.
It seems to me that the simplest approach you can start with is to find a way to emulate the principle of the law of supply and demand.
There is a great video that explains the basis for this.
If managing to implement even the simplest algorithm to cover the basics of that, it might be enough to satisfy your needs.
But as I said before: you should first have your limits well established. How far do you intend to go with emulating the micro and macro economic aspects?