API access is free for development purposes (using the Delayed App Key) for private betting customers and licensed software vendors.
A one-off activation fee of £299 applies for Live App Key access for betting purposes. This is debited directly from your Betfair account once access is approved. Activation fees are non-refundable. Please note: Read only access via the Live App Key isn't permitted.
Please see Personal Betting Access - Application Key Overview for further information.
Which licence do I require indicates the process to follow based on your specific intended API usage.