Better known as the Ultimate Creatures Accessory Set, the prices seem to be dropping significantly since a week ago. MSRP was 19.99 USD back when it first came out (Although my local TRU was listing them for only $4.99). Last week the cheapest pre-owned 6638 was listing for $159.99, and a new sealed one was $600.00 on Bricklink

This one is new, and unopened, for a mere $269.99 USD. It does have a spot of damage on the paper wrapped around it, so it’s not mint condition. Maybe not the one you want if your looking for perfect specimens, but maybe you can hunt down a new banner to tape on if that’s the case?

This pre-owned one is currently on sale for $62.99, and they are accepting best offers if you think that’s too much. Shipping is not included.

Another pre-owned here for $89.95. It advertises “extra brains”, though I couldn’t make out what these “brains” were. It does however claim to benefit charity despite its higher price.

And finally an incomplete pre-owned for $99.99. Although it is missing some parts, it looks like they’ve stuffed some other interesting items that might justify the price.