Good Evening youngster.........I don't know much about banjos...........cept I love the sound........my ?........how hard are they to play and how much do they cost??????
This is probably a broader question than you realize. There are different types of banjos, but I will limit the scope of my answer to 5-string banjos, since they are the variety with which I am most familiar.
You can pay anywhere from a few hundred dollars for a decent "starter" banjo, to tens of thousands of dollars for a coveted vintage instrument. The main concern when starting out should be that the instrument is set up to play comfortably, and sound decent. If the instrument is set up badly, no amount of practice will make it sound "right" to you, and you may even learn bad habits as you try to compensate for the instrument.
As to the question of difficulty, I can only say: If you want to play banjo, you certainly can. Different techniques work better for different people. If you have trouble, try a different style, a different teacher, a different instructional DVD, or even a different hand position.
It takes time, but it doesn't have to be "hard". :)