You might take a look at 'AES's used list to get a sort of high guess as to what a typical radio's "going price" is (or any other list of that sort). From there, it's more a matter of what the thing is worth to you. Naturally, you want to be sort of skeptical about prices that are not sort of close to that "going price". The 'asking' price is a matter of what the thing is worth to the seller, always allowing for the 'bargaining' aspect of the whole business.
Not much of an answer, huh? A '940 would be worth more to you than it would be to me, personal preferences and so on...
- 'Doc