Imagine you were designing your dream house and got an architect's estimate for the building cost, only it was based on a design that lacked wiring, plumbing, and a roof--and assumed you'd need have to pay three different contractors instead of one.

You'd be curious about the estimate; it might offer some hints about how much the house would cost in the end. But you wouldn't spent too much time dwelling on it since, after all, the final price was going to be much different.

Well, that's the same attitude you should take about the estimates of the Senate Health, Education, Labor, and Pensions (HELP) Committee bill that the Congressional Budget Office (CBO) delivered yesterday.

According to the estimates, which Marc Ambinder has posted here, the HELP bill will force the government to lay out around $1 trillion over ten years.  That might not seem so bad; at one time, many reformers feared the number might be around $2 trillion. But, according to the CBO, the net effect of the bill would be to decrease the number of uninsured Americans by only a few million people. There'd still be 30 to 35 million uninsured, compared to 45 millions now.

A trillion bucks to insure 8 million people? Now that seems bad. Really bad. And, sure enough, critics are already seizing on the number as proof that reform will be a boondoggle.

But wait. The Senate HELP bill is not yet finished. Like the house design without the plumbing, wiring, and roof, the HELP bill is missing several key elements that would dramatically change the final estimate.

The most important of these, by far, is the employer mandate--that is, the requirement that employers either pay for their workers' coverage or pay the cost of covering them through a new insurance exchange, set up and run by the government. The HELP bill has a big blank in the employer mandate, because it was an area over which the committees Democrats and Repbulicans were still negotiating when the committee submitted its language.

An employer mandate would change the projections in two ways. First, it would raise money, because of the employers who opted to pay into the exchange. Most likely, an employer mandate would raise between $200 and $300 billion, depending on the details.

The second way an employer mandate would affect the projections is more subtle--but, in some ways, more important. If you make insurance available to everybody through an exchange, fewer employees will demand coverage of their employers (and those that do may not demand it quite so strenuously). As a result, fewer employers will offer insurance in the first place. So you're basically swelling the ranks of the uninsured in one way, even as you're shrinking it in another.

That's what's going on in the partial HELP proposal. A lot of people are gaining coverage, but some are losing it, and the result is only a modest increase in the number of people with insurance.

But if you throw an employer mandate into the mix, everything changes. Now employers have to pay. And as long as the money they'd pay to insure their own workers isn't radically different from what they'd pay to insure the through the exchange, they're probably going to keep offering coverage. The net impact is many more people insured. 

Add an expansion of Medicaid and a well-defined individual mandate--the other big elements missing in the HELP bill right now--and you should get some different projections. My educated guess is something like 35 to 40 million* people gaining insurance, at a net cost of between $1.1 and $1.4 trillion over ten years. Remember, that's the preliminiary estimate CBO delivered a few weeks ago, when presented with such an outline.

None of this is to say there aren't some warning signs in this CBO estimate. Among other things, there's the fact that the law would not allow individuals to opt into the exchanges voluntarily, something that's been a feature of the plans all along but is just now getting attention. But I'll have more to say on that, and other issues, in a bit.

For now, let the architects finish their work. Then look at the price tag.

Update: I originally wrote 30 to 35 million, but a friend informed me that my math was off. Also, I tried to clean up the analogy a bit. Gotta stop writing so late at night. Also, for more, here is Ezra Klein on the estimate itself and Media Matters on the Republican reaction.

--Jonathan Cohn