you pay the fixed price, so if the auction goes to say £3k but the fixed price is £24.99 you pay
£24.99 + p&p + 50p for each bid you entered
the seller gets your £24.99 + p&p + 50p for however many bids the acution in-total has received (some items i have looked at the seller has received over 2k for something thats about £300 in most shops....
you can even bid on bundles of bids which i think is