Does DMing ever make you feel dumb?

I love DMing, but sometimes when my players ask about how big a city is, or when I have to come up with prices on the fly I feel stupid. I don’t know how big an average city is and I struggle with monetary values in D&D. It’s discouraging and frustrating.

I don’t like telling my players “I don’t know”.

Do any of you also struggle with this? How do you handle it?