There are to many factors driving the prices up, baby boomers getting into the market, auction houses (mecum barret jackson), TV shows featuring build up of cars or finding them, as well as the finite amount of cars out there. At best the collector car market will level off, but as the dollar gets weaker people in general look for things to invest in to protect their savings against inflation, I don't see the collector car market tanking anytime soon.... JMHO
I also agree with automan which is another reason it will not bottom out.