Almost all the weddings I have been to were in Central IL. I wish I could have mine there because it is so much cheaper to do things.
Start by looking at ceremony/reception sites. If you can do both the ceremony and the reception at one place it will make things a lot cheaper. Try a public park in Peoria or Bloomington or Decautr. Some of the colleges would also offer nice locations (Milikin, Illinois Wesleyan, and Illinois State University are absolutely beautiful year round. ISU's quad is also an arboretum with every tree native to the state of IL planted there. It looks amazing all year. You could do it in a nature preserve like Starved Rock. Decorate the trees with some tulle and it becomes a really romantic site. Bonus points if you and the bride can find a local to lend you horses to ride to the ceremony on. Then a reception could be done at the ballroom). The most expensive option would be a golf club with banquet facilities. There are tons in central IL, and some will include things like catering or a cake.
If you're looking for cheaper, also think about having the wedding in the afternoon. It's a lot cheaper to pay for lunch than for dinner, but it can still be really elegant. Also only offer a few selections of alcohol or have a cash bar. Some people feel it's rude not to pay for full bar, but at any wedding I've been to down there no one has ever really minded.
Let me know if you need anymore ideas. You can e-mail me from my profile.