How much propellant is required to return the Falcon 9 Reusable's first stage after separation?
SpaceX has stated on numerous occasions that it requires about 30% fuel reserve to return the first stage.
One example from Trent Waddingtons excellent Musk-Stalking site: (Regardless of the URL)
[Question on performance hit for attempting landing the first stage] We effectively lose, in terms of performance... It really depends on what we want to do with the stage if we were to do an ocean landing or a return to launch site landing. If we do an ocean landing, the performance hit is actually quite small at maybe in the order of 15%. If we do a return to launch site landing, it's probably double that, it's more like a 30% hit (i.e., 30% of payload lost).
Further that they planned that into the payload considerations of the booster.
Even further that if that means a payload is too big for a Falcon 9, then the Falcon Heavy is the way to go. They believe it to be worthwhile to take this approach to reduce overall costs, by reusability rather than expending stages.