I was considering something like the Marco Polo but at $200 I have to ask, how likely is it that you'll have a flyaway that goes so far that you lose signal with the DJI Go app? Aren't most downed AC the result of hitting a tree limb or losing a prop? I would think in 90% or more of cases, DJI Go is going to show you exactly where the AC is and you can just walk right up to it. Even if it is out of range, you may be able to reestablish contact by moving in the direction of the flyaway with the Go app open. For me, I look at these "insurance" type things statistically. If there's a 10% chance that on any given failure I may need a tracker, that's worth about 10% of $700. So I'd pay about $70 for a locator/tracker.
Mike