How much to charge for Post Production work ?

AAC

My neighbor hires me to take office party photo for 4 hours.  I can easily quote him for this 4 hours shooting but I have no idea how much to charge for the time spent on post production.  I will select 200 picture among this 4 hours shooting.  Do I need to charge hourly rate less than shooting ?  If so, how much % less ?  What will be the reasonable post production hours to touch up 200 digital files (not all of them require heavy touch up) ?