A few suggestions:
Hotels book up early and can get expensive. Check the EAA Airventure website for housing. Many locals rent their house out for the duration and is better value, especially for a group sharing. AirBnB may also offer options, but best to book early.
Camping is a good option too. $100 would get you tent and bedding from Walmart and leave it at the Goodwill store behind the North 40 when you leave (you might even be able to buy the stuff there in the first place).
Car rentals at Oshkosh are expensive as lots of the fly in visitors book them up. Better to get a car from further away. It's only a 3-4 hour drive from Chicago.