I'm a newbie but I think purchasing your ticket at the airport counter, which avoids the approximately $23 carrier interface charge, essentially matches the discount den pricing.
The CIC ranges from a few dollars up to $23. There are DD $16 flights to Vegas but the normal price is $32 with $10 CIC, so $22 purchased at airport. But the $32 price applies to both Basic and Economy Bundle. The DD cost for Economy is $21, so that's probably a better comparison point. But I haven't had any luck with buying an Economy Bundle ticket at the airport. They would only sell me the Basic. YMMV
Another DD Vegas flight costs $39 (Basic and Economy) vs $47 (Basic). The CIC is $23, so the non-DD airport price is $24. And the Economy DD price is $53 which is more than the normal online price of $47. So in this case, DD is more expensive!?