Rent it for a week and take it back early, and there's plenty of coupons out there to get a discount or an upgrade or two. I can usually find a standard size car at an airport for < $250 a week. Here's a code for National for one weekend day free: NF4861ADM. You have to rent it from 3 to 5 days with a Saturday night included. Comes out to $130 from Lansing Airport from a Thursday to a Monday for a Pontiac G6 size vehicle. Ann Arbor airport has about the same prices. Metro is about $100 more for the same thing.