I am staying on the north side of town, so haven't looked at downtown hotels. Have you tried Hotwire or Priceline? My friends got a great price (on the north side, though) on a room through Hotwire.
We have a social group set up for those of who are going, so come on over and ask there! The name of the group is Midwest Homeschool Convention Go-ers. We're getting together for lunch on Friday if you'd like to join us!