I have the cargo trays and if you're going to put wet and/or dirty stuff back there, the lip on the fitted cargo tray are worth it. If everything you haul is clean and dry, you could save a lot with a basic rubber mat.
I put anything dirty or wet in the box under the floor and prop the cover open. I have a cheap poly tarp folded up under the floor to use to wrap dirty stuff.
I posted a thread suggesting the use of a door mat as an inexpensive option for protecting the floor of the cargo area. Please follow this link to that earlier thread. I provided a couple of photos and a description of what I'd done, and other people offered their own suggestions. It may be a helpful start for you.
It really depends on what you plan to carry back there. If it's usually luggage, golf clubs, etc. the OEM mat is fine. If you carry a lot of tools, dirty or muddy objects, WeatherTech makes a good mat. If you occasionally carry something dirty, do what I do, flip the OEM mat over so the rubber side is up.
I recently had to transport two dirty acid coated deep cycle batteries from our motorhome. I was glad that I spent the money for the Weathertech cargo liner (I bought from the PriusChat store).