When my friend opens presents at her big birthday party, I'm going to announce out loud how much I think each item cost and whether it's cheap or expensive.
Why would you embarrass her like that?
I want to post this stuff on facebook, but I also want to give a reality check to my friends.
You realize you'll be ruining your friend's  birthday? The only reality check everyone will remember how you humiliated your cousin on her birthday. Is that what you want?
No, the kids just don't know what good taste is and they need to realize that I'm a better judge of it than they are.
Shouting out your estimate of how much each gift cost is not going to make anyone believe that you are a good judge, it is just going to annoy them. What they will remember is your behavior and how it made your cousin feel.