You can have a predefined rate card just for a specific client. To create a rate card, go to Contacts > Companies and find the company you need from the list or click on the "+" sign to add a new company.
When you select a company, click the tab Rate cards.
Click the + Rate card to add custom rates for this specific client.
Next time you are creating a budget, you'll be able to add rates from this list by clicking on the New from rate card button.