Of many return flights to Denver, there has only been one time I couldn't get a cab immediately. Usually not a problem. Cost to downtown will be a flat rate $55 or so and make sure they do not run the meter or charge extra for passengers. A town car would be more. For two people there is not significant savings to justify a shared ride service like SuperShuttle. An UberX is about $40.
If you wanted to rent a car a better option to save on taxes would be to rent downtown and return at the airport. Avis, Hertz, Enterprise, and Budget are all downtown. But unless you want to go to the mountains I wouldn't bother and would Uber instead. Downtown parking is spendy.