Good question, cheap vs. expensive! I use a Gitzo GM2541 carbon fibre, which is a $260 monopod, but to be honest I'm not really sure why. It is a stick, no more, no less. Ok, it's light and it has a nice clip so that you can carry it in your belt and the Gitzo twist locks are great, but still. Tripod design and weight is much more important, that you really have to think through before you buy!
So why did I get ther Gitzo you might ask? Well, I guess it's also a question of which brands you have experience with and what you trust! I have always used Gitzo and I love it. In my mind, the extra money is worth using something you know is good in beforehand. However, I am almost certain a Manfrotto monopod for $50 would do the job just as well, or at least almost just as well, but I'd rather stick with something I know... 
I can also add that I do not use a ballhead on my tripod and since I never use a monopod with a lens which doesn't have a lens mount, shooting portrait mode has never been a problem for me.