Well, the part shouldnt be more than 25 dollars. Then you should bleed the brakes.....probably shouldnt take more than an hour. I assume that you wont be doing this yourself. I dont think Id spend more than 60 to 70 dollars for someone to do it for you.....good luck
I had to do this once. It's not that bad of a job if you have some help. You might as well go ahead and change the brakes as well b/c you have to tear the whole cylinder apart. Yeah, and the parts not bad. Around 25 bux if I recall right.