You shouldn't have to pay anything if the O2 sensor is covered. This goes for diagnosing as well.
From:
http://www.epa.gov/oms/consumer/warr95fs.txt
Can Any Portion of An Emissions Warranty Repair Be Charged to Me?
If you have valid warranty claim, you cannot be charged for any
costs associated with the diagnosis or repair of the problem,
including labor charges, parts, or miscellaneous items that are
necessary to complete the repair. For example, if a manufacturer
agrees to replace a catalytic converter under the emissions warranty,
you should not be charged for the diagnosis of the bad converter, or
any pipes, brackets, adjustments, or labor needed to complete the
replacement.