Yes, takes less fuel to land on the drone than to fly back to the launch pad. For larger payloads, it's either land on the drone or throw away the rocket.

It's not just about cost. AIUI, from browsing /r/spacex, it's about feasibility. Less demanding payloads will have enough fuel to return to launch site, some will have enough to land at sea and some payloads will require the use of an 'expendable' configuration where too much fuel will be used to recover the rocket. Expendable launches are more expensive.

Essentially required for high speed launches: https://twitter.com/elonmusk/status/688844190826942465

A bit of a misnomer that rockets go straight up. In fact they go almost horizontal very fast where trying to find a place to land on land makes little sense.

If I am not mistaken, last time Elon Musk mentioned if you fly with used booster, the price go down 30%.

And that's just the beginning.