How to create an increase system + multiplier where every time you buy a slot, price increases [closed]

I need some help with an increase system where price gets higher the more slots you buy
The slots are basically items in a shop. I have the shop and item system already in the website. But there is no increase system so its a new mechanic.

I’ve pretty much gotten this direction:
a flat multiplier, add a field that’s a purchase multiplier to shop stock, grab a user’s total lifetime purchases and do the math

I couldn’t find any info online about a multiplier
Is there someone that can help and maybe walk me though portions of the multiplier so I could understand it? I never dealt with multipliers before which is why I’m stuck.

I can provide code snippets if needed, as I’m not sure what to provide.
This is coded in php/laravel.

Thank you!