While a quality tripod demands quality materials and workmanship, it's pretty safe to say that monopods are nothing more than a camera-mount on a stick.
For this reason, I've been extremely happy with my Manfrotto 679B when I've used it.
It's made of aluminum, has a versatile top for mounting various heads, and extends very far.
I think I purchased it for less than US $50 a few years ago, and it's been a great monopod.
You can put nearly any head on it that you want, and it's very stable.
I currently use it with an Amvona clone of the 486 RC-2 ballhead, and it easily accepts
my Manfrotto 488 RC-2 ballhead, which I find to be quite versatile.
While the 486-clone head may not be up to supporting larger weights, the 488 is certainly up to the task, with the monopod delivering on the stability factor. You're sure to find similar heads to support your gear on such a monopod, if you choose this route.
I've had it fully supporting my Soligor lenses(Tokina-made) of 75-260mm and 400mm designs, as well as supporting a Tamrom SP 60-300mm "dust-pumper" to very good results, and these expirements cover the 486 and 488 heads combined.