How much would you charge for what you do normally but have someone else tag along?
Also, saw some adverts in the metro news paper for extras in both standard and not so standard films music video's etc. Again never having done anything like that before would be interesting to give it ago, not sure even what the time commitments would be.
Just found an interesting article from the times online site about being an extra @ http://www.timesonline.co.uk/tol/money/consumer_affairs/article2109976.ece