The problem with monopods is that as you breathe, it moves not only back and forth, but side to side. However, a monopod is fine when fast shutter speeds are used. I could not afford an expensive tripod, so I bought one for around $150.00. A lot of the newer tripods include a monopod (the head attaches to a removable monopod), so if you buy such a tripod, you have both.
I use a bipod for shooting my rifle sometimes, but while these stop lateral movement, they don't stop the up and down movement created when you breath. If I have to use my bipod for hunting, I still rest it against a tree trunk to form a sort of tripod. Otherwise there is too much reticle movement (up and down), specially if I get a little exited. Biathlon shooter slow down their heart rate to control movement, but I don't know how to do that.