With the rays all coming out of one of two export locations, I'd call a pretty locallized area. The world is a pretty small place with the ease of communication we now have, so I'd bet that everyone down there knows what everyone is charging whether they actually discuss it or not.
And as far as a free for all on the rays until they're all exported - that would just create havoc, with no one caring how they got their rays or what condition they were in when they were shipped out. Every exporter would just be in a mad rush to get in and get out with whatever they could. Having controlled numbers means that they'll take better care of the fish they do get, because they'll want to get top dollar for every one they sell.
And Why would the Brazilian gov't/IBAMA care what the market price for them is?