As the system gets gamed, the costs will be much, much higher than the CBO is estimating.
Arnold Kling explains his words in more detail. Elsewhere:
If you think of the social cost of this bill it's well below $900 billion. If we could collect in tax revenues all the dollars in savings and new wages that people will get because of this bill, it would bring the cost well below $900 billion.
Jonathan Gruber explains his words in more detail.