How expensive is it to go to London?

My dad works for an airline, so the plane ticket is not a problem. My mom tells me we can go to London, but she says that it is expensive. I have been on Expedia and found nice hotels for $100 a night. My mom's only fear is how much food is. How much is McDonald's in downtown London? Can you eat fairly cheap? Can I compare this to New York City or Chicago price wise? Any help would be great.