A bit of digging suggests the number originated in a press release from Budget Committee ranking member Judd Gregg (R-NH), written the morning after the CBO unveiled its analysis. It reads "American taxpayers are about to see an unprecedented expansion of the federal government that will cost a staggering $2.5 trillion when fully implemented."
The underlying critique has a sliver of merit to it. Democrats did indeed diminish the cost over the first 10 years (2010-2019) by delaying the tax-and-benefit provisions for a until 2014, long after the bill becomes law. They felt as if they needed to push some key reforms down the road to keep the bill's CBO score from exceeding Obama's $900 billion top line. Hence Gregg's caveat "when fully implemented." But even if you were to start the clock in 2014, and stop it in 2024, the number $2.5 trillion seems to have been made up entirely out of whole cloth.
And, of course, this critique elides the fact that, whatever the federal responsibility for health care becomes as a result of this bill, the total package is projected to dramatically reduce the deficit in both the near and long terms.
Then again, I guess Democrats probably don't mind too much when John Ensign is presented as a prominent face of the Republican party.