I think using keyword planner to budget is presumptuous too. If you had to have a number and coming in blind sure; but discovery is sooooooo important.
Ask them important questions about what this will do to their business. Find out what their revenues are (directly or indirectly) and ask them if they want to drive topline rev and grow. Do they want to make sure margins hit their P n L?
I think this does two things. Demonstrates interest and maybe some knowledge of their business. Separates you from other PPC houses that leverage common approaches. Maybe you stand out on that alone.
Come with a POV sure; come curious you’ll have a leg up.