Paget's disease is an intraepithelial cancer
of the nipple, meaning that, in and of itself, it is in-situ cancer. The prognosis has to do with the underlying cancer that is present elsewhere in the breast ~75% of the time.
Therefore, if there is only in-situ cancer elsewhere, the chance for cure is 90-100%. If invasive cancer is present elsewhere, the cure rate is stage-dependent.