A common question posted to the View-Master discussion group is "How much is this old View-Master stuff I found/inherited worth?"
As with many collectables it all depends on the details of what you have.
If you are looking to sell your View-Master stuff, your best bet to get a good price is selling it via eBay. There are many View-Master collectors who browse there often.
If you really need a price guide I would recommend:
Brad Welsch, "Collectible View-Master" prices
Available for $19 here - www.amazon.com/exec/obidos/ASIN/0967719704/ref=ase_stere3dbooks3dbo
Available for $30 here - http://www.vmreels.com
John Waldsmith "Stereo Views" prices
Old version available for $19 here - www.amazon.com/exec/obidos/ASIN/0870695789/ref=ase_stere3dbooks3dbo
New version available for $$17.50 here - www.amazon.com/exec/obidos/ASIN/0873494091/ref=ase_stere3dbooks3dbo
Harry zur Kleinsmiede, "View-Master Reels and Packets" - no prices
Available for $58 here - http://www.stereoscopy.com/3d-books/view-master-reels-guide.htm
Mary Ann & Wolfgang Sell "View-Master Memories" - no prices (reference)
Available for $30 here - http://www.cinti.net/~vmmasell/memories.html
http://www.ebay.com - prices
http://www.3dstereo.com - prices
http://www.vmreels.com/ - prices
http://www.fisher-price.com/us/view-master/ - no prices (reference)
The most common questions are about the most common viewer, the Model C. This viewer is very common because it was so well constructed. It's worth in the $7 to $12 range.