The Congressional Budget Office is suggisting that a rescue of Fannie Mae and Freddy Mac is about 50% likely, and could cost $25 billion.
Me, I would put the chance of needing a rescue at over 90%, and multiply by 10, or $¼ trillion for the cost to the tax payers, just based on my gut.
As to secondary costs rippling through the economy? I’m not even going to try to guess at that.