If building a supersonic passenger plane with an existing engine was viable, it seems like somebody would already be doing it. Which means it's probably not viable, in which case Boom is an aircraft engine company first and foremost; once they have the engine, wrapping a plane around it should be the easy part, relatively speaking. Developing a new engine costs on the order of $1B, so the development cost for the whole plane would probably be between $1.5-2B. So they'd pay for that with their first 10 planes to Virgin, with maybe some money left over to change their name.