S.T.
For my 2 kids, I pay $10/hour for irregular babysitting (i.e. date night, etc.)
My neighbor paid $15/hour for her kids to be babysat regularly once a week. My girlfriend would only pay $10/hour for babysitting for her kids twice a week and she's had a real hard time finding and keeping someone.
You also may want to determine if you're going to pay "under the table" or take the childcare tax write-off. This can have some ramifications on how much you pay.
Based on your 2-3 hours every other week, I think you would do well to pay $10/hour for a responsible High School aged individual or older.