Set appointment prices depending on the hour of the day with the Special Hours add-on (available here). The settings are specific to staff member providing a service.
Compatible with Bookly Appointments plugin starting with v11.6.
When the client has chosen an hour from time slots available, the total amount due will use the price that you have set up for appointments booked for that hour during the day.
Set it up in Staff Members > Services and add a Special Hours block that will be different from the default price set up for the service in general (and copied by default to the staff member's page) or for the service provided by the staff member.
Make sure you're comfortable getting around staff's individual pricing settings.
GOOD TO KNOW:
You can also set up limited availability of services at various hours during the day, with the Service Schedule add-on.