I cannot see from your profile where you are, but I know that one of the clubs where I teach has a deal where people with a certain kind of health insurance have a very inexpensive membership. Harris is right that it would pay to contact fitness centers that are close to you to see what deals they offer. You can also ask your insurer for a list of their options.
Also remember that many Ys have funds set aside to offer financial aid to those in need. If you ask the question because you are held back from joining due to the expense it is worth calling your local Y and asking if they have someone who can talk to you about aid.