First, they do want revenue neutral plans-- that's one thing killing single-payer. Even if single-payer pays for itself overall, they don't want to shift the payments to the government. Really stupid, but apparently this is the case.
Second, they are very concerned that cost containment occurs. Initial estimates that an early version of the HELP bill would not hold costs were used by Republicans to hit them over the head. MSM were gleeful . The Republicans will use it again. Ironic, that they may help force the public option that is anathema to them(although none or few will vote for it). A national Co-op would only contain costs if there are increased regulations to hold down costs. The needed regulations would be considered anti-capitalist by some, but I think they could be effective. It's not clear they are being considered though. For example, CAPPing costs (via premiums) that insurers could charge patients. Here's one way to do it:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=389x6313878