There's probably not anywhere on the coast where you can't eat for $30 outside of some of the Casino steak houses, but if you're going to have drinks or appetizers, you'll hit $30 pretty quickly at Chimneys or mary mahoneys. Phoenecia is basically the same, but you can bring your own alcohol, so there should still be plenty of options that will let you stay under $30.
If you're staying at South Beach, I'll second Shaggy's. It's not fine dining, but the food is solid, reasonably priced, you'll have a great view, and this is a good time of year to go (it should basically be open air when you go). You could get mahi sandwhich, appetizer, and hang out and have some beers and still stay under $30 I think.