View the client's profile, and scroll down to Services. Then click "+ Add special price" at the top right corner of that Section.
A window will open up where you can start typing any service and product. Similar to Google style search, the options will pop up depending on what you input. In the example below, we typed "dog walking" and selected the specific Service we are looking to modify pricing for the client in question.
Using the example above, after changes are saved, if any pets under this client are booked for a 30 Minute dog walk, the estimate and the final prices at time of invoice will respect the special pricing which overrides the standard ones defined in your services listing for 30 Minute Dog Walk.