As he died in 1999 anything you can get of his at a reasonable price is getting hard to find. I collect him specifically and there are copies of some of his work out there with good copies of his signature and they are asking high prices for them ie $US1000+
So any genuine piece is worth having. Hint, look for a label (check the labels file above) and forget the signatures he only signed his very best pieces and they are multiple thousands if you can find them.