That's how I've been getting a few prices, so far.
I go and search a particular piece of equipment and get prices for three or more similar items from eBay then average them. I also take a look at KEH and B&H. I try to make a judgment of what is the TOP price a particular camera might bring. That's just my starting value.
Then I grade the camera on its lens, film path, body, etc. on a scale of 1 to 5 and come up with an overall average. Let's say a camera's overall score is 3.5 out of 5.0. That's 70%. So I figure the camera is worth 70% of the top price.
Finally, every camera or non-trivial piece of equipment is going to get a manila string tag with its item number on one side and a short description on the other. Once this is all done, I'll be able to decide which cameras I want to keep, which ones I want to fix up or clean and use and which ones will go to new homes.
Basically, I'm only a third of the way done!