Beginner in Boston, MA
You can also learn to sail and race on the Charles River for $100-150 (I think) with unlimited time on the water. Great and inexpensive way to learn the fundamentals which will make you that much more appealing as crew. The harbor courses are good, but a lot more $$$. You could try calling the harbor clubs to see if you can work in exchange for classes, but I'd think that's more of a long shot. If you are serious then I'd suggest spending the money on classes and then crewing.