$30 is fair for the original retail version complete(disc, insert + manual, no cardboard box). Its not rare, but it is one of the most sought after CD games.
The Duo pack-in should go for $20 for the disc or $25 with the manual tops.
The Turbo Ys games market on eBay is controlled by a few losers trying to rip people off. But you can still find them at decent prices fairly regularly.
Although Ys III complete with cardboard box is usually advertised for $99, I bought it mint complete + cardboard box not too long ago for a Buy-It-Now of $35.