From personal experience:
"But I saw you do that in 15 minutes!"
No, you didn't. I spent nearly 300 hours on this presentation so that I wouldn't waste your time.
"Can you do it for 20 bucks?"
You see that idea I wrote? That's 20 bucks worth. What you want is worth way, way more.
And on and on.
Trust me, I'm a doctor.
Haha, urgh... countless times... countless times.