I just got one recently.
It cost me about 140.- delivered from Trackballworld; it's a little less money if you go without the two sockets for extra buttons on the back-- the Massdrop model.
I used to use the Sanwa Pro trackball, a Japanese import that costs about 80.- delivered, and it's much smoother rolling mechanism, but I don't like the shape and button layout quote as much. I assume I will get used to it.