Gopro distributes through a dealer system that strictly controls price. You aren't going to see many discounts or sales of authentic Gopros from authorized dealers. The way some dealers compete is with "bundling" the camera with accessories. You can find gopro 6 for about $325-$350 on ebay but these would be grey market, refurbished, or potentially counterfeit products.
As for accessories, it depends on what you want to do. Given the forum, I'll presume underwater video. Most of the mounts in accessory kits will sit in a drawer. If you use a tray you'll likely need one mount that has a 1/4-20 tapped hole. The tray may even include it. Poles often mount directly to case. You'll probably want a red filter. You can post-edit for color correction and get good results but that takes time and knowledge. Most want something out of the camera that is good enough to throw on youtube and good filters will do that. Beware cheapo Chinese red plastic disguised as a red filter. Polarpro is ok, Backscatter is best in my opinion. (They won't be $22).
You can recharge batteries in camera but I prefer a separate charger. I change batteries each dive even though I might still show 50%. I would rather have extra battery power than not enough. I have a three-battery charger and three batteries. I've had good luck with Wasabi.
