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?"