Bavauto shows an IPOD interface for $139 beans which means you should be able to find one for $100 or less elsewhere. Rooz mught be a good source too.
Addition: One of my cohorts just e-me'd this site:
www.discountcarstereo.com
said they have a couple of "Blitz safe" adapters available and aren't going to "cripple" your wallet (aka Crutchfield)