Determining the value of a Polaroid camera can by tricky. It's best to think of these cameras as having value as a collectable and value as a camera. Roll and Pack film cameras are collectable curiosities which can fetch prices between $5 and $50 (and more), depending on how desperate the collector is to obtain the available camera. SX-70 and later cameras can fetch $20 to however much a buyer is willing to pay.
Here is my take on collectables:
Probably one of the better indicators of value is to monitor Ebay closing bids on your target camera. For instance, if you are looking for a model 450, search for "Polaroid 450" and watch the items and take note of their closing bids. Bear in mind that a seller may ask whatever he chooses for a camera, but it's the closing bid that reflects the value. Many Polaroid cameras listed on Ebay will never sell because the starting bid is too high. To get a picture on closing bids, follow the auction and check the bids after it has ended. The closing bid, if any, will be indicated on the item page. Be aware that there are sellers who "bid" for their own auctions with other user accounts to make it appear that an item sold for a much higher price than it's worth.
There are a few models which are worth more than a few dollars, though pack film cameras may see downward price adjustments in the next few years since they will no longer be able to be used as cameras. The venerable models 180 and 195 list for over $1000 on Ebay at present, since they were much sought after for their manual exposure control and excellent lenses, but I would expect that buyers who want to actually use the camera will no longer be interested in purchasing it for lack of film and the only buyers left will be collectors. SX-70 foldable cameras can fetch over $200, and SX-70 hard bodied models $25 or more since the Impossible project still produces film. 600 series cameras can go for $20 or more, and Spectra cameras from $20. Expect a wide range on prices since your average seller often thinks that these unique cameras are worth much more than the market delivers.