$500 or so seems to be the going rate for an untouched byron sabre. It varies. A built one may go for LESS than that, even with all the equipment in it.
If you are thinking about just going turbine...skip this plane. By the time you have gotten a good OS91, pipe, and byron fan, it's another $400 plus. You don't need brakes. Standard servos are fine for this plane. But unless the thing was a real steal, or you happen to own the engine and equipment that goes into this plane already, you are looking at $700-1000 to get into the air. Save that for a turbine instead.
If you want to get your feet wet, find a built one...there are so many bargains around in the way of used DF stuff...I would not pay more than $500 or so for a Byron Sabre WITH all the gear in it.