Would you be able to accommodate a live-in nanny where you live? Generally speaking, the "room and board" that you provide as part of the compensation makes a live-in nanny less expensive than a live-out nanny, who is compensated solely through a salary. It is also very convenient since you don't have to get your kids to/from a childcare provider outside your home. Look at nanny ads on Craigslist or in the Chicago Reader, and place ads of your own in both as well.
Another affordable alternative is like a live-in nanny, but there are set rules governing the arrangement - i.e., an "au pair" to whom you would serve as a host family through the U.S. State Department's "Au Pair Program." (See http://exchanges.state.gov/jexchanges/programs/aupair.html.) With your soon-to-be-newborn though, you would not be eligible for an au pair until your baby is at least 3 months old, unless you or another adult is at home. Last I knew, the weekly salary was very affordable, relative to other forms of childcare - it is fixed by the program, and the au pair's hours are limited to 10 hours per day and a maximum of 45 hours per week (you pick the schedule, which you are free to change around on a weekly basis). The au pair must have at least 1.5 days off, one of which must be a Saturday or Sunday. The au pairs are foreigners between the ages of 18-26 who have gone through the au pair training program and are available through a handful of private agencies in the U.S. who are authorized by the government to administer the au pair program by placing au pairs with U.S. families through a mutual selection/matching process. You pay the agency a fee, and they provide you with information about their available au pairs from around the world. The au pair can likewise learn about your family based on the information you provide in your application.
Good luck to you. I hope you find a childcare arrangement that works for your growing family!