Thanks, I really love figuring things out when I can't immediately see a proper solution, and you're totally right about trying to find ways to make it more approachable. Unfortunately, even the first equation is something that you can't reasonably do yourself without a calculator that can at least do summation.
Due to how the price changes with level it's difficult to make a concise equation that most people will understand. If it's of any help, I'll link the calculator and label everything you need to know (It's WAY easier to use than to understand.) If needed, it wouldn't be too difficult for me to just put the values in a grid and just let people look at a chart and trust the numbers.
Edit: This is the calculator all you need to enter is the stone rarity, the level you want to reach, and the level you're starting from.