Best way to set up a spawn area

Currently, I am using a plane and base the area on scale x=20 z=20 and then I get the bounds of the mesh to create a random position to drop items.

I saw a video using an edge also.

What is the best way performance wise to do this. I like the plane because I make it a child of the prefab to determine it's drop locations. Plus I can view it in the scene for debugging.

But, does a plane get to expensive and is there a better way?

I can see me going crazy and having 20+ or so small planes in a game level.