[PK]

For the record, though it introduces significant complexity, I agree that it's probably fair to give a reduction for the extra SBDM (Strength-Based Damage Modifiers) that had to be bought for Power Blow. I'd figure out the SBDM cost without Power Blow (let's call that [X]) and then the increase in SBDM cost when adding Power Blow (let's call it [Y]), and then charge the player for [X + (Y*0.8)]. It's a weird way to apply a limitation, but it's going to be more fair than any other method.

(Please note that it is only Power Blow for which I would do this. Not for anything advantage-based, etc. Power Blow is kind of a crappy way to increase your ST, so I can understand the need to address that here.)