Charging for photography


I recently did a photoshoot for a school Yr6 leavers party (They all had 1 photo taken which will be printed - this is included in the price they paid for the party)

However, they all had loads of other pictures taken with their friends etc. So I was going to put them all on a CD and sell them (I did the photography free of charge as it was my neices leavers party) as I would like to make a little money.

How much would be reasonable to charge for a) a single image emailed, and b) a CD full of all the images?