Yesterday, Speaker Nancy Pelosi (D-CA) told reporters that the House version of the health care legislation will cost under $900 billion and reduce the deficit, claiming to have estimate from the Congressional Budget Office (CBO).

The CBO, in its score of ObamaCare in the Senate and in previous preliminary scores, pointed out any budgetary savings aren't a given:

The net cost of the coverage expansions would be more than offset by the combination of other spending changes that CBO estimates would save $404 billion over the 10 years and other provisions that JCT and CBO estimate would increase federal revenues by $196 billion over the same period. In subsequent years, the collective effect of those provisions would probably be continued reductions in federal budget deficits. Those estimates are all subject to substantial uncertainty.

let's examine the record of Congressional forecasters in predicting costs. Start with Medicaid, the joint state-federal program for the poor. The House Ways and Means Committee estimated that its first-year costs would be $238 million. Instead it hit more than $1 billion, and costs have kept climbing.
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Medicare has a similar record. In 1965, Congressional budgeters said that it would cost $12 billion in 1990. Its actual cost that year was $90 billion. Whoops. The hospitalization program alone was supposed to cost $9 billion but wound up costing $67 billion. These aren't small forecasting errors. The rate of increase in Medicare spending has outpaced overall inflation in nearly every year (up 9.8% in 2009), so a program that began at $4 billion now costs $428 billion.

The Medicare program for renal disease was originally estimated in 1973 to cover 11,000 participants. Today it covers 395,000, at a cost of $22 billion. The 1988 Medicare home-care benefit was supposed to cost $4 billion by 1993, but the actual cost was $10 billion, because many more people participated than expected. This is nearly always the case with government programs because their entitlement nature—accepting everyone who meets the age or income limits—means there's no fixed annual budget.