Going Rate for Babysitters

My husband and I attended the school fundraiser on Friday evening.  Before dad got home, I asked my 6th grader what would be an appropriate "payment" for a co-babysitter who is babysitting a well-behaved brother for a max. of six hours.  Her reply, "How about twenty bucks?"  My reply, "For you both to split?!?!?"

The funny part about this is that I read my daughter's "social media" page.  She was bragging how all she had to do was to sit on the couch and snap together the ocassional Lego to keep her brother happy.  I am quite certain that they did absolutely nothing while we were gone (I was back in four hours) except watched TV, ate snack, and surfed the internet.  When I called, I told them they should have their brother go upstairs to bed.  The last hour, they didn't even have to pretend to do anything.

What WOULD be reasonable compensation for this?  Could I write it off as "family obligation"?