I think that is a good idea especially when you just want to test a skill. There are so many skills, that if i want to test all of the on the highest level you'd need more than three characters, and not everybody wants to level multiple characters again and again.
Maybe where you actually can degrade a skill and the better you are in that skill the more expensive it gets.
(CharLv x SkillLv x ChargeStage)
Char Level is clear. Skill Level is when you put 5 Bronze charges and 3 Silver ones it's 8 AND Charge Stage is
2 for bronze, 5 for silver, 8 for gold, 10 for ultimate?
I'm Level 3 and have a bronze skill lv 2 which i want to reduce for 1 so it would be (3x2x1)=6
But if I'm Level 15 and want to change a Silver charge of level 3 down to bronze it take three steps
(15x7x5)+(15x6x5)+(15x5x5)=525+180+150=525+450+375= 1350 in total
The System could be a lot more refined, but it's the idea that caiomelo12 had that i really like.