It should be quite easy to fit. I helped with one on a 70hp Yamaha at the weekend. Simply put, you drop bottom of the leg off the engine by undoing all the bolts (there may be one hiding under the anode as well). The impeller will probably be in a little plastic or metal box that the drive shaft passes through on the bit of the leg you remove. When you take the casing off, it can just be slid up the shaft and replaced.
I wouldn't have a clue how much one would cost, somewhere between three and thirty quid, there's not much to them.