Appropriate Allowance for Kids

By Packmatt on Flickr
By Packmatt on Flickr

Now that my son is in middle school he is starting to do more things with friends like eating out and going to the movies. We have never had an specific allowance for him, but it is beginning to be clear that we need to give him his own spending money. Any ideas on how much money is appropriate? Do others use allowance as payment for chores?