The econ lots are $7. Depending on how long you're going for, Amtrak from downtown is $6 and the lots there are $3 or $4 a day (can't remember which). Or from those same downtown lots, $1.75 on the city bus to airport.

Ultimate cheapest is to park at a park n ride which allows overnight parking, then bus to the airport. $1.75 each way but not all county lots allow overnight parking.

Best lot for free parking to airport:
http://www.dot.wisconsin.gov/travel/.../d2/pr4010.htm
From here it's one bus (#80) at the corner of the lot (NOT the flyer that runs into the lot). Straight to the airport.
More info: http://www.ridemcts.org

PM for add'l details.