I typically pay around $250-$300 for the OMA-DEN roundtrip (nonstop). Despite the presence of Frontier (F9) on the route, it has always been much more expensive for me to get to DEN on UA than to ORD.

Here's the FareCompare airfare history chart for DEN-OMA .

There have been some occasional sales on F9, UA, and NW down to $160-$175, but I seem to remember it being mostly "leisure" fares (Sat. night stays, etc) and not helpful for my Mon-Fri/midweek typical business travel.

NW isn't very convenient because it's a connection through MSP.