Montana has a cost of living index of 97 relative to the US average of 100. This is close to the national average. Montana 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 Montana's median income of $75,340, that is $1,883,500. However, if you plan to spend more (or less) than median income in retirement, scale the number accordingly.
Montana taxes income at 4.7%–5.65%. In FIRE, your primary income comes from portfolio withdrawals, dividends, and capital gains. Long-term capital gains in MT are taxed at the same rate as ordinary income, including IRA withdrawals. Factor this into your withdrawal rate planning.