From Sagada, you can go back to Manila via Bontoc and Banaue.
Take a jeepney to Bontoc. From Bontoc, take the Cable Tours bus going to Manila.
You may also take a bus or jeepney to go to Banaue first, then from Banaue, go to Manila.
You may also want to try the Sagada-Bontoc-Baguio-Manila route.
Or you may decide to take the Sagada-Baguio-Manila option.