Appears to be $20 registration fee per car and driver.
As of 6-June the registration page only has 2019 information.
Application
Each entry includes one (1) MUSHROOM FESTIVAL ADMISSION WRISTBAND , one (1) Free Tshirt* and one (1) Dash Plaque* and a coupon for a free pack of mushrooms. Additional Mushroom Festival Admission Wristbands can be purchased at the Festival.
Article: The Mushroom Capital Of The World Might Surprise You