Octane itself is not tied to any 3d software package, so the price (199$) has no relation to the Poser market. The Poser plugin is set at 99$ and this is identical to the plugin price for Maya. I think the prices have been set by Otoy.

The price for the additional hardware depends on how much speed and resources you need. I spent over 500$ on a new videocard and felt it was worth the investment.

Is there a market for it at these prices? There are many poser users who spend a lot more money on content every year, so if they think that this render engine will improve their renders, they may find it worth the money. Best thing to do to see if this really worth your money is to download the trial versions and see for yourself.

Compare it with a tool like ZBrush. For some people this tool is invaluable, but for other users Poser's morphing tool is more than enough and think it is overkill at that price.