I think it depends more on time and ancillary costs than the number of photos. In other words, a set figure per photo may not be the best way. With wedding photography there's the time needed to discuss the clients' requirements before and after the event, plus the (high) cost of albums, mounting and so on, and you may well need an assistant who wants paying.
On the other hand, if you're commissioned to photograph some event/exhibition or whatever, you can reel off a few hundred photos in a couple of hours. There's the processing time on top, but if the client is happy just to receive a CD of JPEGs you wouldn't expect the cost per photo to be anything like as high.
I reckon that if you can make around £200-£300 per day that's not bad, especially outside the south-east.
