Actually, the higher airport costs add quite a bit onto the cost of a ticket; the average cost per enplanement is close to $20 (and this does not include the passenger facility charge of $4.50, the highest allowed by Federal law). Of course, costs need to be high, given that the airport is carrying $4 billion in bonded indebtedness.
I think Continental pulled a fast one on the DIA planners and United, actually. CO was the first hub carrier to sign on to the plan to build DIA (and that's why they ended up in the more convenient Concourse A). Both CO and UA had been reluctant to move from the far more convenient facility at Stapleton and had been dragging their feet on the plans for DIA. I suspect that when CO management made the internal decision to pull down the DEN hub, they decided it would make sense in the long term to basically make United pay for the cost of Denver's boondoggle on the prairie. So, CO bought into the DIA plan (and negotiated an exit clause in the lease), which allowed the plans for the new airport to move forward. Faced with the facts that the new airport was going forward and Stapleton would ultimately close, United was forced to make a commitment to DIA as well.
There's a pretty good paper on DIA at: