UIUC is an in-state school for me while UMich is out of state. According to a net price calc., I’ll be paying roughly 18k yearly for UMich compared to maybe 5-10k yearly for U of I (not exact bc. I’m only a soph in HS but I will probably get lots of merit from UIUC).
Get into both programs first - it is way too early for you to compare cost. So many things can change between now and when you get your admission decisions.