Reddit reviews Torker Unistar LX 20" Unicycle Black/Chrome

You can learn on a cheap unicycle, but if you can spare the change I'd get a Torker Unistar for 100-150 depending on size etc. That will assure you don't have some of the problems common to lots of cheap unicycles (cottered cranks, uncomfortable seats, etc.)

I've bought two on Craigslist, one was 80 bucks (24") and one was 50 (20"). Both were in good condition, though one was assembled incorrectly (cranks were on the wrong side). I also picked up a cheap 20" for ten dollars from a local sporting goods store - that was a good deal because it was so cheap but I have to hammer the cotter pins back in every once in a while, it doesn't have a quick-release seat post, etc.