Someone may be able to give more up-to-date info than me, but last I heard/read, they will reimburse you for parking only up to the amount they would have paid for you to park at your base. So, if you are based at IAH and their deal with IAH was for $40/month, and you have to pay $50/month to park at Spiffytown Municipal, you'll be reimbursed $40.

Of course, I haven't been a commuter in a while, so I don't know if this is still the policy.

That makes me wonder, if you are based at an outstation that has free parking, and your home airport has paid parking, would this mean they would not reimburse you at all?