Conclusion 1: For a single boost, the cost is base value * multiplier for the boost level
There is a base value for every boost, and then a multiplier for the amount of bonus. The multiplier is fixed for all boosts.
Example base values:
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MoC is the highest I've found; climb/swim the lowest.
Multiplier table:
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Note the multiplier is a seed 1 summation.
So, e.g. +6 Pole is 240 * 21 = 5040 points to recharge.
There is also a minimum recharge cost of 1000 points.
This holds for Pole, CM, MoC, and swimming (all bonuses) and STR stat. I welcome others to replicate/invalidate the findings on other skills/stats or to extend the work to skill ranks/stat bonus.
Edit: The numbers flow much more naturally if I double the base and halve the multiplier compared to what I originally posted.