Just to loop back ... the trip went well. There is a flat rate either way from downtown Denver to the Airport - $55.57. Taxis are easy to find and readily available at both ends.
Alas, neither the Four Seasons nor Ritz-Carlton (a dated former Embassy Suites) gave us a good night's sleep due to a party at the former, and omnipresent heavy cleaning supply aromas in the room at the latter.