Back in 1999/2000, I remember seeing one of these go for ~$500 on yahoo auctions. That was the one and only time I saw it.
I never really understood the purpose of the Power Console. When I first saw it, I thought it looked cool, but it was announced and shown at the same time the SuperGrafx was unveiled. They only announced like 2 games for the SGX at that point (Battle Ace and Daimakaimura).
It was clear that the Power Console was intended to be used with Battle Ace at that time. It also cost more than the SGX itself which was pretty expensive already.
I don't know how many actually exist (or existed), but I wouldn't be surprised if there were only 1 or 2 of them made. However, it was supposed to be released at the same time as Battle Ace and from what I remember, it was never really cancelled. It was pushed back to the point where it was never shipped. So there might be a warehouse full of them somewhere!