Unless you're going to theater (off off bway) and/or dinner downtown, your best bet to get from the theater back to the Waldorf is probably just to walk it. Otherwise, a taxi won't run you all that much ($7/8 each way), although you'll need 2 taxis for 6 people. But, for that distance and # of people you'd only be saving a couple of dollars total by using the subway and it's not worth the inconvenience imo (esp. if you're slightly nervous).

For eating - you can easily get lots of good meals for under $20/person. I'd suggest you walk along restaurant row and look at the menus one day or do a search online before you go. Best options will probably be family style eating where you share dishes. You'd probably find Carmine's in midtown fun for your group (Italian food) and should be within that budget.