There is no extra cost for the booking service. The Client Booking service will be available for Pro plans and above, but for the next 45 days (until September 24th) it will be available, at no extra cost, to ALL accounts.
There are no fees associated with the Client Booking service other than standard credit card fees for online payments made by your clients. Currently the credit card fee is 2.9% and 30 cents per transaction which will be taken by Stripe before they remit the remainder on to your bank account that you have setup with them.