There are two types of taxes placed on marijuana: state and local.
Both occur only at the retail level on end-user purchases. State taxes are set by the Montana Legislature while local-option taxes are optionally enacted by counties.
State taxes are set at 20% for adult-use sales. Local-option tax rates are set by the county up to a maximum of 3% on medical, adult-use, or both. These taxes are in addition to the state taxes. You can expect to pay 23% in taxes.