Audax UK charges organisers £7 to register an event, plus the cost of brevet cards and validation fees etc.
From the Audax UK Organisers Handbook:
"To calculate your entry fee you will need to consider the following expenditure:
Audax UK brevet card & validation charges. Current charges for the 2016/7 season are:
Brevet Cards (Black & White) 35p [minimum charge of £7.00]
Validation Fee 20p (For each rider completing the event)
• Hire of start/finish and control venues
• Catering at start/finish and/or controls
• Your own organiser’s expenses e.g. for route checking etc.
• Expenses for your helpers
• Printing route sheets etc.
• Donation to charity/club funds
Aim to set your entry fee to make a small surplus based on your expected number of
number of entrants; and related to the costs of putting on your event and the facilities offered."
Also a small additional charge is made to cover Paypal fees.