50 Year Composition Roofing goes for $200 Per Square.
Tear off of existing & Haul Away goes for $40 Per Square.
More if you have 2 layers of roofing.
If your existing cedar shake roofing has "spaced sheeting" underneath it, It will have to be covered over with solid sheeting (plywood).
If you are going to undertake this adventure, I would suggest using the plywood sheeting with the Radiant Barrier to keep most of the heat out of your attic and house.
Hope this helps.
ps: a guarantee is only as good as long as the company is still in business! beware of long term guarantee
Adders will be replacing any rotted underlayment, rotted fascia, ice/water barrier, lots of stepped flashed walls, valleys etc. extremley steep pitches.
If you are going to put a 50 year shingle make sure the flashing used will last at least as long (aluminum coil stock probably won't)