Not all that much, really. An almost-empty rocket stage is very light, and it doesn't take much to slow it down from terminal velocity (which can be rather low

Not all that much, really. An almost-empty rocket stage is very light, and it doesn't take much to slow it down from terminal velocity (which can be rather low if you fall right) to 0. In fact, one of their main problems seems to be throttling down an engine enough that it produces less thrust than the weight of the thing. I once did some rough numbers to guess that the landing propulsion will take 10-20s with 1-2 of the motors, using 5-10% of the fuel at launch. Hair-raising, but well within the capabilities of today's sensors, actuators, and computers. The extra fuel is not so bad on a first stage, which is not so weight sensitive as higher stages. And it may be about what it would re-enter with anyway, given reserves. In which case, it's a pretty good deal. A pinpoint 5mph dry landing is a lot nicer than a 30mph wet one, which it what you'd get with parachutes, and it only costs you the weight of legs, since all the other engine and guidance stuff has to be there already.