For those of you with a car, here's a handy site. NYC Parking Garage shows daily and monthly rates for many many garages in Manhattan. You can find garage by selecting a neighborhood, putting in an address, picking a cross street, or choosing an attraction. And then type in when you're parking your car and voila, you'll get a map and list of the different rates.
The site has a disclaimer ("although we strive for 100% accuracy, it is an unachievable goal on an island with approximately 1000 parking garages and lots"), but it's certainly useful enough to see that in many cases, if you're willing to walk a bit, you can save a lot of money. The next stage for this map would be to have a mass transit map layered over it, one could see if parking is convenient to subways or buses.
We plotted some rates for a Friday night 6PM-11:59PM visit to the Theater District (let's say you're doing the dinner and theater night out).