The CBO score for the Finance Bill is out [pdf]. Here's the best paragraph in the document:

The proposed co-ops had very little effect on the estimates of total enrollment in the exchanges or federal costs because, as they are described in the specifications, they seem unlikely to establish a significant market presence in many areas of the country or to noticeably affect federal subsidy payments. As a result, CBO estimates that of the $6 billion in federal funds that would be made available, about $3 billion would be spent over the 2010–2019 period.

Co-ops are so ineffective you can't even give away the start-up money. As our ads said, the co-op idea doesn't work.

Igor at Think Progress has more on the CBO score.