American Airlines has “basic economy” round-trip flights from Milwaukee (MKE) to Aspen or Vail (ASE/EGE) for $264 with a connection. Flights January to March, on Tuesdays and Wednesdays.
Note: Carry-on bag is allowed for free. Seat is assigned randomly for free at check-in. Upgrades into “Regular Economy” are available for +$30 each-way.
Note: United is matching with their version of “Basic Economy” which you should avoid due to it now allowing a free carry-on bag.