Camry Airbag sensor

All:

My mom has just been told that she needs a new airbag sensor at a cost of
$650.00.  The dealer won't budge on the cost.  Is this a common problem on
Camrys?  It is a 2000 model year with 40,000 miles on it. My mom likes the
car, and it runs great, but it did have to have the block replaced at around
30,000 miles ( these thinks sometimes happen, even to Toyota!)  After the
dealer replaced the block, the car ran fine.

Has anyone else had to have this sensor replaced?  What did you pay, if you
don't mind me asking?  Should I go to a private garage?  Is there any danger
in not replacing it??

Thank you,

Joe R.