According to both Murphy and personal experience, things take longer and cost more than planned. Why do you want the bike? In general, you will be lucky to recover your parts cost, and none of your time in a restoration, unless there is a known demand.
So is this just a project? A long-time dream? Or a plan to make some money? To answer your original question - I have no idea, because I don't know the BSA market nor the parts availability. Typically, the better shape and more complete the project is to start with, the better the chances of making or saving money by the time it is done.