Considering the amount of labor involved, plus parts, I don't think that they are trying to rob you. Totally rebuilt with a brass sleeve, plus a lifetime guarantee sounds great, provided they are still around years from now if it wears out.
Personally, I would first look for a good used one.
Just out of curiousity, dosen't NAPA or O'Reilly sell rebuilt master cylinders? I purchased one for a '52 Ford 3/4 ton a few years ago
