I'm an avid user of www.glitchremote.com which covers not only where things are, even in your home street or for sale via SDB.
It covers auction prices, and its a quick way to find anything you need urgently in Ur. Cant remember where the foxes are? search by location. Cant be bothered to brush the foxes and just want the cloth? seach home street SDB's or the auctions. :)
*edit* although the rarest items like the glitchmas yeti are so rare that they are practically priceless. They wont show up on glitch remote becaue they are not traded often. The only details of when items have been traded are via the market place forum where people have put up a large selection of items just for the one yeti.
There was a market post with some suggested prices for cubinals which were hotly debated. Other than that I don't think there is one. I've heard 400k or a little less for the dolls. Things like the Glitchmas Yeti would go the tens of millions IF you could convince the seller to part with it at all. You'd need some creative ooomph similar to what ZombieKitty did to get hers.
I recommend ecurnomics.com. It will give you a history of the past auction prices for any item. It is not perfect for this use. I don't think it includes Towers and many super high ticket items are not sold though auction. But it is the best tool out there for establishing value and it is extremely easy to use. I love the zoom feature where you can blow up one part of the timeline.