When I was flying down to MN, I have the option of MSP and 3 hr drive or Sioux Fall, SD and a 1.5 hr drive and the Sioux Falls flights on UA were in the US $600-700 range booked 2 weeks out and $1300+ booked 1 week out or less.. Some flight prices that are limited and low volume can be pretty expansive since cheap seats fills up early. I ended up flying mostly via MSP and visit some friends during the overnights.