All my exercise is basically for free - I take stairs, go on walks, run a few times a week, and lift light weights. No gym membership and no classes.
For some people a treadmill can actually be a very good investment in the longer term - that is, if you use it!
I agree with Dr. Beckerman. For 53 years, my exercise has been "free" and I am at the low end of my healthy weight range. Jogging, walking, race-walking, exercising at home (I use books with callisthenics and weight bearing exercises) are all free. Books can be found low cost at bookstores or used bookstores and even on eBay. A jumprope is a helpful way to get QUICK cardio exercise, but work up slowly. Using your arms and legs during arm circles leg lifts provides weight "resistance" training without investing in dumbells. If you wish to purchase hand weights or stretch bands to use at home, stores like Walmart in addition to Sports stores carry such equipment at very low costs. Yard sales are an excellent source of used exercise equipment! Have fun with exercise, if possible, so that you will look forward to it! Cheap DVD's (or old used video tapes) are also available, at a very low cost.
There is a site that you can use that sets up a home plan for you. The site is http://www.freetrainers.com. It's a great tool and you save all the money that you would put into the gym :). I hope this helps.