I just sold my NordicTrack SL710 Recumbent exercise bike through Craigslist. This is a great bike because I can use it anytime I want but it is too big for me and my legs are short so I have to drop the chair all the way down to the handler bar. This is a combination of home and gym machine. The lady who bought it will have her knee surgery next month and this bike is good for her. I sold it for $100. I am going to get an upright bike this week, of course used exercise bike.
There are lots of exercise bike that you can buy online, you can check Craigslist, Ebay or amazon or from your local sports store. For me, it is better to buy slightly used machine so you are not paying too much for it. You can also check Nordic Track exercise bike online. You can also give this as a Christmas gift.