This is not how people usually do it, but I belive that you should charge a standart price for a amount of exp. Because pokemon have different amouts of exp needed to reach a certain level. That way, you can check in game how much xp the pokemon has and how much it needs to lvl 100, then charge based on that.
Based on how much other people charge in this forum, I think a good price would be 157 $ per experience. That way, you would charge 148k to lvl up a pokemon from the medium fast category from lvl 50 all the way to 100, for exemple. Which sounds like a fair price
This table might help you: https://bulbapedia.bulbagarden.net/wiki/Experience#Experience_at_each_level