[PvK]

The formulae I derived from looking at Version 1.5something wasn't geometric (no ^2). It added a linear amount to the increment at each step:

Low - each level adds Cost x 1.
(5000, 10000, 15000, 20000, ...)

Medium - level 2 cost twice level 1, then each level thereafter costs (Level - 0.5) x Cost.
(5000, 10000, 22500, 40000, ...)

High - each level adds (Level - 0.5) x Cost x 2.
(5000, 20000, 45000, 80000, ...)

In other words, Low is linear increase, Medium increases at each level by a linear amount, and High costs twice as much for each level (after the first) as Medium.

At least, that's what it looked like to me.

PvK