call me old fashion, , but I would try to make the repair. First, remove the button on the top of the knob. Second, remove the screw from the center of the knob. Third, pull the knob off. If there is a piece of plastic left behind then you only need to go to one of the large home repair supply stores and compare your's to the ones available. Reverse the removal proceedure...you're done.
If there is no plastic, you will have to turn the water off, then remove the valve retainer and pull the valve insert out, then take that to your home repair supply center, match it, reverse the removal process, and your still 42 bucks ahead ...not including the plumbers fee (more like 80 bucks an hour.