Facepainting $$$

Facepainting gigs... How much do I charge? I want to offer a competitive price thats fair to the consumer as well as my fellow painters. I`ve seen painters asking from $2/ face to $15 at festivals, whats the going rate on that kind of painting? Also, for example, a child`s birthday party... Would that be a flat rate, or a rate then per kid? or a corporate event, flat rate + per person? I`ve thought about this for a long time and have decided to ask for some advice. Any would be great!