Personally I am happy with everything I got for the money I pledged ($600).
But looking at Steam I do think 9.99 for a the source hunter DLC is pretty steep. You get a few in game item and then a few goodies next to the game like a digital art book. I would expect much more for 9.99. As a consumer I would feel ripped off for that.
Why would anyone feel ripped off by it? If they don't think it's worth it, they wouldn't buy it, it specifies exactly what it includes beforehand. If nothing else, it's a way for big supporters of the game to give Larian some extra money. Surely as a $600 KS pledger, you should understand that there's people who want to do that.