Wisconsin's cancellation fee is $5.00.
Additionally, from the WI DNR Site:
If you cancel no later than 3 p.m. Central Time two days before the day of the reservation, you'll be credited a full refund of camping fees, but not the reservation or cancellation fees.
If you cancel after that, you'll be credited a refund of camping fees for all but the first night. You will not be refunded the reservation or cancellation fees or the first night's camping fee.
No refunds are made for any part of a three-night holiday reservation once the site has been occupied.
I do know for fact that if you have to check out early on non-holiday weekends they will refund your unused nights. We had to do that a couple of times.