Hey Ceric, if the code is on the paperwork for the vehicle, I wonder if it is possible to program 2 key's using the supplied code without having to replace the ecu.
I bought my cx-9 used, but the seller provided all the paperwork for the vehicle from when it was new, which included a factory "checkoff" sheet if you will, and this "checkoff" or "spec" sheet has a 4 or 5 digit programming code (for the advanced keyless system) printed on this paperwork, and states on the paper the importance of not losing the piece of paper.
Julie, if push comes to shove and you have to end up buying a new computer and 2 cards&keys, have the dealer order a new computer, but I think that buying new cards&keys on ebay is not a bad idea if you really are in a tight spot with money. You can get a combo advanced card brand new with a brand new auxkey for about 70-80 bucks. (I got one for 60 brand new, new card and new aux key by doing the "make an offer" on ebay)
I have programmed an aux card that has the picture of a sedan trunk rather than a liftgate like the CX-9 has, with no problems... which I suspect is an advanced card for the Mazda 6 or RX-8 or what have you.. from everything I have seen, all the advanced cards/credit card size advanced card whether they be for the 6, the RX-8, CX-9, all share the same fcc id. If you see a brand new combo on ebay, for a Mazda, and it is the size of a credit card, then really it should work. Buy 2 of them. This will save you a hundred bucks, maybe 200 bucks on that quote of 680.
Look, if worst comes to worse, and the dealer says that your ebay keys&cards are no good, then you can utilize ebay buyer protection and get your money back from ebay, but I've bought 1 used card and 1 new card, several aux keys on ebay, and never had a problem. That is until the service manager dropped the smart card onto the concrete. (that card now works intermittently after bing dropped by him, and this is now the one that stays in my lock box) (I also did have a knock off aux key from ebay where the RFID chip fell out of the key because it was a knockoff and only had one spot of glue holding the RFID chip from falling out)
I only had 1 card when I got my CX-9. I now have 2 in use, and have a 3rd in a lock box so that I will never run into this problem.
Your missing card has to be somewhere....I'd spend 8 hours searching for the missing card before I would pony up 600 bucks.