The Budgetary Implications of NASAs Current Plans for Space Exploration (PDF), Congressional Budget Office
"In response to a directive in the National Aeronautics and Space Administration Authorization Act of 2008, the Congressional Budget Office (CBO) has updated its 2004 report analyzing the budgetary implications of the National Aeronautics and Space Administrations (NASAs) vision for the nations space exploration program."
"According to NASA, its current plans will require an average of $19.1 billion of funding annually from 2010 through 2025, with the Constellation program accounting for about half of the total by 2017. Under its current plans, the agency also intends to conduct 79 new robotic science missions through 2025, requiring funding of $4.7 billion annually, and to perform aeronautics research, at a cost of about $460 million annually."
Editor's note: The COB looked at 4 scenarios. No matter how you look at it, at current spending levels, NASA will continue to have program slips and will have to make cuts to other programs to keep Constellation anywhere near on schedule. COTS-D doesn't seem to have entered the COB equation:
Editor's Update: A couple more stories and commentary related to this.
Lawmakers Pressure NASA to Delay Shuttle's Retirement, Wall Street Journal
Former NASA chief moves on--NASA not so much, Scientific American
CBO costs out various NASA budget options, Space Politics
Scenario 1: Keep Funding Fixed and Allow Schedules to Slip
"By CBOs projections, with such cost growth and fixed annual budgets for the Constellation program, the initial operating capability for Ares 1 and Orion would be delayed from March 2015 to late 2016, and the first mission to return humans to the moon would be delayed from 2020 to 2023"
Finally, for this scenario, CBO assumed that NASA would forgo any missions on the space shuttles launch manifest that did not occur by the September 30, 2010.