As this thread has stuck around, some of the prices have increased:

RTD Skyride to downtown is now $10. Service is hourly except during the morning and evening rush

Flat rate taxi from the airport to downtown is $55. Flat rate from downtown to the airport is $51. There should be no additional charges for people or baggage. Make sure they run it flat rate as the meter rate would be $65+ depending on traffic. If the meter comes on at all, make sure to ask about the flat rate. The flat rate boundaries are posted on a sticker inside every cab. There are also higher flat rates to Boulder, Broomfield, and the Tech Center.

I believe Supershuttle is $21. However, as it circles the loop of downtown hotels you could end up spending a lot of time in the van. That said, you can wait in front of a major hotel downtown and the Supershuttle is by every 15 minutes. In the same lane as a Supershuttle, during peak times, you can usually find some unmarked white vans that can negotiate rates. I once got a private ride to the tech center area for $35 and downtown for $40+tip.