Around $80 for a quite ok one like manfrotto. Get a tall one with 3 sections. Avoid cheapos. Many do not use a head with it, but I find using a head more suitable for me esp in nature shots.
Use it to shoot sports, fast events and some nature shoot. I like it even better when the subject is angled high up on a tree up or low down (river animals, insects, butterflies). It gives me more confidence as my hand is less tired/shaky as well after a long day. My pics come out better with it. You can read up the details in dpreview and other reviews.
the monopod unlike the tripod has only one leg so it gives up some stability for faster deployment. Helpful especially if u are on a tour overseas and may want to walk and shoot with some support without having to deploy the tripod, shoot and keep it only to find that you want to use it again a few metres ahead. Helpful in crowded events too.