Good luck - there is a fair bit of money to be made at such events and if you are a half talented phtogographer with a decent amount of sales skills this really can be a gold mine.
If you are looking at event photography you should look at the cost of
hiring / buying a dye sub printer and selling shots on the night. Apparently it's more profitable than simply shooting and selling via the internet. Basically it turns in to an impulse purchase more so if the there is alcohol on tap

.
Also if you read up on it there is potentially greater profit to be made if you print all the photos and sell the prints rather than viewing on screen and printing to order. All down to the psycology of people actually being able to hold and see the finished article before handing over the cash.
Base your prices on what the event is with black tie being more expensive than a children's show but single photos between £7 to £15 are not uncommon. Remember that 3 for 2 type promotions are always a good way of getting people who would normally buy 1 to pay for 2 (the cost to you of printing 3 rather than 2 is marginal).
Just remember to tell the tax man and that equipment costs are offsetable against tax though an accountant will help here.