That question is very difficult to answer. In the past couple of weeks, ebay prices have shot up ~20GB models have gone for anywhere between $600 and $800, and there haven't been enough 60GB sold recently to say.

But, at the time that you were originally buying, I'd guess at $400-$500 for a 20GB, and $600-$800 for a 60GB, but again, there's probably more fluctuation in the pricing of the 60GB models as fewer have come up for auction.