Why is Wolf's Rain so ridiculously expensive?
You're looking at the original run of Wolf's Rain, which is no longer in production, and as such is subject to secondhand supply and demand price increases.

I recommend that you look into the Wolf's Rain - Complete Collection Limited Edition DVD Set released in 2005. This set started at 149.99 and is listed on the related auctions on the link you provided to eBay for $195, a ~25% increase from its original price.