“Come on! What do you say?”
You meet a girl who you'd like to go out with at a dance club. You ask her on a date, but she doesn't answer. You want to encourage her to answer, so you say this.
Come on! What do you say?
Ask this question when you've made an offer to someone, but they're hesitant to take it.
For example, if you're trying to sell your old bicycle to a friend, you can say:
I'll give it to you for a hundred and fifty. What do you say?
Use "What do you say?" after you've made your offer, but before the person you're talking to has given their answer. If they've already said "yes" or "no", it doesn't make sense to ask "What do you say?"
"Come on" is something to say when you're trying to persuade someone:
You also say "Come on!" when someone is upset or angry, and you're trying to get them to calm down:
Come on. Don't talk that way. Of course I love you.