Online prices will always be cheaper, especially here in NYC where the dealers charge 150% of MSRP. Even if mazdaswag is half price, you needed someone to install the parts so look at it that way. Labor for one axle probably is about one hour, if it's done correctly with cleaning and fresh brake lube. Did they give you the MV pads they took off? You paid for them.
Would you care to name this dealer so other people can avoid them? It's not defamation if you have the receipts and proof. You might also want to contact Mazda national customer service and perhaps they can do something to maintain customer loyalty.