Is University of Illinois Urbana-Champaign or Michigan State cheaper for in-state students?

The biggest practical tradeoff is that these schools only offer in-state pricing to residents of different states, so there is no single student who would be in-state at both. If you are an Illinois resident, UIUC is the in-state option and Michigan State would be out-of-state. If you are a Michigan resident, Michigan State is the in-state option and UIUC would be out-of-state.

Because of that, the cheaper school for an in-state student is almost always the public university in your own state. In-state tuition at a flagship public like UIUC is typically far lower than what a nonresident would pay there, and the same is true at Michigan State for Michigan residents. Once you add housing, meals, and travel, the home-state school usually still comes out much less expensive overall.