How the restaurants are in Managua depends on what you like. If you want just "American" food you can find everything from McDonalds to TGI Friday to Buffalo wild wings. And at most of those places prices are a little lower than in the US. However, if you are willing to try someplace that looks different than what you would see in the US, you can find a lot of places here where a couple can eat and have a drink or two for under 20 USD. The only place I had "problems" (if you know what I mean) after eating was at a place called Burger King. But after a few days, I managed to leave the bathroom and resume normal activities.
One piece of advice is stay away from the lettuce and raw cabbage. And don't eat at places like Mercado Huembes, either. (I have seen tourist guides that recommended eating there!)