The government bailout of Fannie Mae and Freddie Mac could end up costing twice as much as first thought.

The latest government estimate shows the bailout will cost tax payers $259 billion. So far, those mortgage lenders have received $135 billion.

In comparison, the combined bailouts of financial companies and the auto industry cost about $50 billion.

Not only did those bailouts cost less, the Wall Street bank bailouts have actually earned a $16 billion return for taxpayers.

On Thursday, the government provided a broad estimate of the costs of bailing out Fannie and Freddie. The final cost will depend on the direction of home values over the next few years. If prices fall sharply, as some analysts forecast, Fannie and Freddie won't be able to recover as much money on foreclosures. They would require more taxpayer aid.

The best-case scenario assumes home prices remain flat for the next two years.

"If the economy does unravel in the next couple of quarters, then the costs will mount very rapidly," said Mark Zandi, chief economist at Moody's Analytics.

Thursday's estimate was the first time the housing agency has released a public estimate of the taxpayer tab. The combined bailout of the two mortgage companies is on track to be the largest of the financial crisis.