Simple question:
Can a quarterstaff double as an arcane focus?
The D&D Basic document lists 'staff' under the Arcane Focus section in the equipment list. So yes.
Matt Petruzzelli @mpetruzz Jul 28
Does an arcane focus staff also/always count as a quarterstaff? @mikemearls
Mike Mearls @mikemearls
@mpetruzz believe so, yes. i think that's why the quarterstaff became a one-handed/versatile weapon
The quarterstaff listed under weapons has a price of 2sp, while the staff that's an arcane focus has a price of 5gp. They aren't the same thing. On p. 48 it says an arcane focus staff is "specially constructed," which would explain the difference in price. Whether or not you can use an arcane focus staff as a quarterstaff in combat, the rules don't say. I'd certainly allow it, but I suppose that technically by the RAW, you can't.
Make a quarter staff that is a focus too for 5gp 2 sp.
This was on twitter a while back.