Every 2 levels in a Skill from Fortune/Reputation/Spirit = +1 in cost for the skill in question.
E.G, Level 0 & 1 = 1 Fortune/Reputation/Spirit
Level 2 & 3 = 2 Fortune/Reputation/Spirit
Level 4 & 5 = 3 Fortune/Reputation/Spirit
52 trillion, I'm just barely big enough to fit the entire screen but big enough to where I can stay underground and go on 20,000+ combos. Also, I idle and leave it on, lol.
If you use abilities effectively, your progress should be a little faster than before (even considering increased improve costs).