A lot of the info comes from the press releases surrounding this book (unfortunately, I have to rely on second hand accounts of the book since I still haven't been able to put my hand on a copy):
http://www.businessbookgp.com/
and 100+ other websites and blogs talking about F1. Basically it's an estimation based on the figures that seem to come back the most. It IS an estimation, and estimations can be wrong, sure.
I actually did make a specific thread for this discussion now, so if you have any better or different info, please do state it there. F1 financial waters are not the clearest, so the whole point here is to gather as much info and as many "trained opinions" as possible in order to end up with a better idea of the current budgets than we have now.
So yes, you can roll your eyes all you want, as it is a starting estimate, not an absolute truth. But you could also help the research effort instead of just bashing freely. If you have a better idea than me about the 2011 Mercedes GP budget, why don't you state it?
Up to you.
And to answer your question more directly about the Mercedes budget, the 2010 one was announced by Mercedes in a press release. At 80M Euros.
And another Mercedes press release said they would reduce their budget by 10% this year in agreement with the spirit of the RRA.
If you want, I'll try to find these press releases again and post them in the other thread.