If you don't mind installing it yourself you can get them from multiple dealers online shipped for about $210 - $250. It took me about 40 minutes to install it and that was mostly me being anal about cleaning the gutter well, torquing everything right, etc. That said, Mazda does MSRP them at $300, and many dealers offer them only at this price point.
Long story short, if you want the dealer to install it then that is not a bad deal (though I feel like with a bit more pushing you could just get it included, but that may not be worth the time and effort). If you are okay with self-installing it then I would pass and order one from one of the dealers that offer them for cheaper online.
CX-3 GT AWD Dynamic Blue Mica