Here is the skill sheet, with almost every variable easily changed. Feel free to play around!
The basic equation I used involves a cost per total level added to a cost per skill level. This allows scaling across individual skills without penalizing the player for the order in which the skills are selected.
Base Costs for Health, Damage, Mana: Base
Multipliers for Level and Skill: MultSkill, MultLevel
Log Bases for Level and Skill: LoglevelLb
Exponent for Level and Skill: ExlevelEx
Total level at next upgrade: X
Skill's level at next upgrade: Y
(MultLevel * XlevelEx
(X) + 1) + ((YskillEx
(Y) + 1) * Base * MultSkill)
Edit: I guess I could explain a little more
Download the excel spreadsheet (need Office for this part) Then, change around the red values up top. The green values at the bottom are the total cost for the build written to their left. Bases are the 8,4,4 for the first upgrade (written as 7,3,3 because of the +1)
The exponent section dictates how quickly higher levels become more expensive, and you can customize overall level growth vs individual skill level growth.
The Log is used to "soften" the exponent in the early levels and serves to incentivize a certain cap. It is a standard Log10
right now, as that breaks at level 10.
Edit: Added skills to level 50, updated image and .xlsx
Edit2: Added multipliers for both Skill and Level costs, as per Purlox's request
(His equation would be: ExLevel = 1, MultLevel=2, MultSkill=1.2)