Illinois has a cost of living index of 93 relative to the US average of 100. This is close to the national average. Illinois offers a typical cost structure — housing and expenses track closely to US median.
The 4% safe withdrawal rule suggests a FIRE number of 25x annual expenses. At Illinois's median income of $72,200, that is $1,805,000. However, if you plan to spend more (or less) than median income in retirement, scale the number accordingly.
Illinois taxes income at 4.95% flat. In FIRE, your primary income comes from portfolio withdrawals, dividends, and capital gains. Long-term capital gains in IL are taxed at the same rate as ordinary income (but retirement distributions may have different treatment). Factor this into your withdrawal rate planning.