Most ethnic restaurants throughout New York can provide you with good choices (a banquet even) for under $50, but there are so many choices. An excellent guide is that of Robert Sietsema, The Food Lovers Guide to the Best Ethnic Eating in New York City. He rates restaurants, and his three star restaurants are usually terrific.
Another possibility is to purchase Zagat's. They list restaurants that have dinners for $35. Some of these are from the among the best NYC restaurants. Often the menu is limited, but you get a chance to explore the chef's cuisine in the same environment as everyone else. And you will meet your $50/person goal with tax and tip. I don't have my NYC Zagat's by me, but this way you can choose a restaurant that seems to have your favorite cuisine nearby - you do have to wade through the verbiage in Z's reviews. I think the Union Square Cafe has such a deal and so does Aureole, but I believe 50 or so restaurants are listed.