It's a proactive adjustment for NEW cars only. They ARE NOT mandating that cars be retrofitted with the equipment. The 250 million car number, therefore, is incorrect and thereby makes your cost analysis incorrect.
On another note, it's not only lives saved. My friend just a few weeks ago had $3000 in damage done to her car in the lot at work because some jacktard hit her car backing out (and then drove off...) which might have been avoided if the camera was there.
It might prevent the needless deaths of children and might also prevent some smaller claims (helping reduce insurance for all...maybe). Since we are in refusal to increase the skill requirements for licensing, I think overall this is a good thing. In the end, most drivers are actually quite horrible. [and as always, 50% are in the bottom half of skill level]