Where a default price is set for a client
When viewing the client details, you will see which price book is set for the client under their settings:
Change a client's default price book
To change the client's default price book, click on the EDIT link in the top-right-hand corner of the Settings box (under the client's details).
Once you click EDIT, you can select that client's new default price book.
Remember to click on the Update button to save the change.