I too think that over $400 is pretty pricey. It might pay to shop around a little. I had this going on for quite a while on my Porsche911, and I don't think even its MAP or O2 sensors are that high.
However, in my case, the valve guides were pretty much shot, so it cost me about 6 grand for a whole top overhaul. Hope you get out of this for a lot less than that!