As I said earlier, his Distortion pieces are one-offs and the high end of his work. They are rather expensive.
But I've only ever bought his work new, in Crieff. I don't know anything about the secondary market.
The bowls are newer than the massive vase I have, but I do know that is fairly early. I did just go and check it - no date is written beside the sig, but I have dug in my folder and found the reciept for it. It was bought in 2004.
If you look at the link to his work in the Strathearn Gallery and click on a piece, the price new, will come up with the enlargement. Or you could contact him at his website and ask for the price of a distortion bowl.