I think this information is confidential. And every rider has a unique value calculated from the wind tunnels that they used. We are only left with what is published. We can only guess estimate from looking at their position and comparing it to riders that have published the data before. there are also some values out there from some specific papers but you have to pay for the information.
One way of calculating the CdA is during training several times and comparing it with the power tab. I guess this is the way that some riders use when they can not afford the tunnel.
This are the numbers that I had originally from a book:
Drag Factors
-------------------------CD-----Area--------CDA
-----------------------------------m2------------
Vertical commuter--1.10----0.51--------0.56
Climbing------------0.99----0.44--------0.43
Racing--------------0.88-----0.36--------0.32
TT-------------------0.69-----0.36--------0.25
Drafting------------0.50-----0.36--------0.18
These are the numbers from a paper:
Other Drag Factors
-------------------------CD-----Area--------CDA
-----------------------------------m2------------
Vertical commuter--1.15----0.55--------0.632
Climbing------------1.00----0.40--------0.40
Racing--------------0.88-----0.32--------0.282
TT-------------------0.69-----0.32--------0.221
Drafting------------0.50-----0.32--------0.160
Moser Bike---------0.51-----0.42-------0.214
Frost Values, CdA:
Boardman-------------------------------0.180
Indurain:------------------------------0.24-0.25
Estimated for Cancellara------------0.23-0.24.
Actually mine estimated for Cancellara with the basic formula was around 0.25. But just by looking at his position it looks like Frost numbers are better for him.
Here is a link given by Frost. There are some basic information about positioning:
http://www.biketechreview.com/performance/pareto.htm