Security at the last royal wedding in 2011 between Prince William and Kate cost nearly £6.4 million ($8.7 million), according to the Press Association.

The government provided the police with a special £3.6 million ($4.9 million) grant to cover officers' overtime pay.

However, that wedding was in central London. Prince Harry and Markle will be married at Windsor, on the outskirts of the capital city. That should reduce security costs.

Regional authorities are paying for security-related infrastructure, and the local police force will bear the cost of policing the event.

"If ... there is an opportunity to recover cost this will of course be explored," said a spokesperson for the local Thames Valley Police, who declined to provide a cost estimate.