Airport-owned long term is $6/day; off-airport sites start at $7/day. The shuttles for the airport-owned long term lots are few and far between, so if you park there, leave extra time.
We went to the private lots some time ago because we hated the wait.
I do have some good news for you, however. I know how you can get great parking for 7 days free, provided you are not going w/in the next 6 weeks or so. Go to
http://www.parkexpress-stl.com/ and register at the site for their mailing list. They will send you a welcome letter containing a card good for 7 free days. The normal daily charge is $8/day or $7/day for stays of over 7 days.
This garage is at I-170 and Natural Bridge Rd., so I know it seems far away, but it is the most well-run airport garage I have ever seen. The staff is REALLY helpful, and the shuttles come every 3 minutes, 24/7. The shuttles take the back route into the airport, and they are very quick. They also have complementary soft drinks and coffee in the waiting area, which is a godsend when you are headed for a hot car.
If you are coming all the way from Springfield, have you considered a park/fly hotel deal? I know that some of the airport hotels offer up to 2 weeks free parking in their lots if you stay the night before you fly out.