Adding a service where the price may vary in salon according to time taken is no problem. If you're not familiar with how to setup services, click here to see the initial steps

If you price by the hour, simply set the application time as 01:00 hour and the price as your hourly charge, and then click "display as from price" at the bottom. 

You may also wish to include that the price is per hour in the service name, like below. 

When it comes to booking in the service, you can simply adjust the duration and price to suit the client's needs.