I have struggled with this in the past. The key takeaway I've learned is that the person is just trying to relate a feeling to you - about what they are talking about - e.g. not to convince you. Stop trying to be a know-it-all or Mr. Logic.
Example: Woman friend enters wearing thrifty-looking but interesting pea coat.
Friend: "Hey, check out my coat. Guess how much I paid?"
Me: "Hmm. $25?"
Friend: "No. $30"
Both: Silence
What I should have said: "$100?!"
Or even better?: "Let's go case out that store. I want one in my size." (joking)
It's not about getting the correct answer - this is not a game show "The Price is Right" - being friendly is more about letting someone express their emotion behind their words and maybe even brag a little