The Osmo Action is priced on par with other similarly spec'd cameras, but is a bit more than your generics or lower specification action cams. The normal retail price is $499 and for that you get a 4k 60FPS capable camera with DJI's own image stabilisation called RockSteady. Of course you can drop the resolution and frame rate down for better battery life and smaller file sizes but this is what the camera is capable of.
A GoPro with the same capabilities will cost you $599 for the GP8 (admittedly this has their latest tech in it that the Osmo doesn't have so it does have some extra features but comes with a higher price tag). The GP7 is $529 which is more expensive, though you can find bundles with extra batteries, memory cards, or a host of other accessories for the same price or cheaper than buying the camera and accessories separately.
Even the DJI website will run free postage or deals offering the camera with some accessories for the standard price of $499 or the camera itself for $399. These same deals are available at many brick and mortar stores.
The Osmo Action is compatible with the full range of GoPro accessories and has the same mounting bracket as the GoPro. This means if you own a range of accessories you won't need to go and buy a whole bunch of new ones, or if you have bought some generic ones off eBay, these will work also.
The genuine accessories are not overly expensively comparatively speaking. Spare batteries will cost you $29 each and come with a handy plastic case which will also store a memory card (see the video >>>). If you want the charging hub, this comes with the hub, USB-C cable, and two extra batteries for a bit over one hundred bucks.
In the box you will get the jacket or housing for the camera, one battery, the camera, and some mounting brackets with 3M VHB tape for securing them to a variety of surfaces. The default accessories are enough to get you on the water if you are looking to hat mount.
One thing to remember though is that you will need a Micro SD card for recording. Unless you buy a bundle with this in it, you won't have any memory for storing shots or vids unless of course you already own one. Something you need to check is whether the card is up for the job. When recording in 4K @ 60fps the bandwidth needed is very high so make sure you have a card that can handle the data throughput. Cards above Class 10 are usually needed for this type of recording.