You just fly back to the launch area. It's immanently feasible to fly that route and always have a buffer of assurance that the rocket's trajectory will not head towards populated areas. If, for whatever reason, that changes then they press a button, the rocket zips apart and falls in pieces into the Atlantic ocean.

As for cost, fuel is cheap, it's one of the cheapest things in the whole operation. Think about the fact that every commercial airliner has the capability to make an unpowered landing and to save fuel doing so, why don't they? Because operational complexity adds cost much more than using up fuel does. By the same token, the greatest costs in orbital launch come from throwing away the hardware after every flight and operational complexity. If using extra fuel enables them to more easily reuse the stages and get more launches out of them as well as reduce overall operational complexity, then it makes economic sense.