Let's do a quick review of rail fares to airports. Using the trip planners on transit agency websites, this is what I get:
Code:
CITY AGENCY MODE FARE TRIP
Atlanta MARTA Metro $2.50 Airport to Five Points
Baltimore MTA LRT $1.60 BWI to Lexington Market
Baltimore MARC Rgnl $4.00 BWI to Penn Station
Cleveland RTA Metro $2.25 Hopkins Airport to Tower City
Chicago CTA Metro $2.25 O'Hare to Clark/Lake
Chicago CTA Metro $2.25 Midway to Clark/Lake
Minneapolis Metro LRT $1.75 Lindbergh Terminal to Nicollet Mall
Philadelphia SEPTA Rgnl $6.25 Airport to Suburban Station
Portland TriMet LRT $2.40 Airport to Pioneer Square
San Francisco BART Metro $8.10 SFO to Montgomery Street
Seattle Sound LRT $2.75 Seatac to Westlake
St Louis Metro LRT $2.75 Lambert Main to Convention Center
Washington WMATA Metro $2.35 National Airport to Metro Center
Washington MARC Rgnl $6.00 BWI to Union Station
Here's the same list, re-sorted from most expensive to least:
Code:
CITY AGENCY MODE FARE TRIP
San Francisco BART Metro $8.10 SFO to Montgomery Street
Philadelphia SEPTA Rgnl $6.25 Airport to Suburban Station
Washington MARC Rgnl $6.00 BWI to Union Station
Baltimore MARC Rgnl $4.00 BWI to Penn Station
Atlanta MARTA Metro $2.50 Airport to Five Points
Portland TriMet LRT $2.40 Airport to Pioneer Square
Seattle Sound LRT $2.75 Seatac to Westlake
St Louis Metro LRT $2.75 Lambert Main to Convention Center
Washington WMATA Metro $2.35 National Airport to Metro Center
Cleveland RTA Metro $2.25 Hopkins Airport to Tower City
Chicago CTA Metro $2.25 O'Hare to Clark/Lake
Chicago CTA Metro $2.25 Midway to Clark/Lake
Minneapolis Metro LRT $1.75 Lindbergh Terminal to Nicollet Mall
Baltimore MTA LRT $1.60 BWI to Lexington Market
The interesting thing here is that after San Francisco, all of the regional rail connections are then clustered at the top of the list. They range from $4-$6.25.
Based on precedent from other cities, Denver's airport train should probably have a fare somewhere in the $6 range. RTD might be able to get away with a little bit more, but anything above $8.10 would be setting a record, and anything above $10 would be unheard of.