WTB Dark Gray or Black 04-09 Cargo Cover

You should check eBay periodically. Be flexible on your searches, e.g., search "Prius Cargo Cover", "Prius Tonneau Cover", "Prius Cargo Shade", etc.
In the past, people used to be able to find one for about $100.

I just did a search of eBay, and this is the lowest priced gray cover I found (but there's a tan one available for $99) -- it's Buy It Now for $169, or Best Offer:

eBay Motors: 2004-07 TOYOTA PRIUS REAR CARGO SHADE COVER GRAY COLOR (item 400008520247 end time Dec-11-08 14:31:42 PST)

Why not offer the vendor a price you're willing to pay?

Good luck.