As someone who has been buying and selling vintage models (a lot of Aurora) for quite a while, I can personally attest to the fact that trying to set a value on a kit can be agonizing. Setting the value on a diamond, for instance, is fairly easy. There are accepted guage parameters to go by such as color, size, clarity, etc., and depending on where the stone falls, you get an accurate value (assuming the person evaluating knows his stuff). No such luck with model kits. In our hobby, the intrinsic value of a kit is mainly determined by individual perception. Sure, condition matters, but more importantly, as stated by John, the value is determined by what the individual feels it's worth. Has the modeler been looking for the kit for years, does he need a particular kit to complete a collection, is he going to keep the kit as a collector's item or is he going to build it? Just a few things that effect value.
I usually spend quite a bit of time researching a kit to try to determine its value. The problem is, the prices swing wildly and it's sometimes impossible to reach a conclusion. The Collectors Value Guide is by no means the final word on value. It was written using as much objective data as possible, however, is still very much just a guide. I wish I could tell you how many times I have put a kit up for auction and was totally blown away by the final selling price, both very high or very low. I have seen models up for auction in pathetic shape...broken boxes, missing parts and decals, partially built, partially painted, that have commanded prices that made my jaw drop. Just depended on how badly the buyer wanted that particular kit.
Basically, I have learned to trust my instincts and experience. After I finish with my research on a kit, I set the value accordingly based on a very educated guess. I find I'm usually in the ball park. Anyway, what I am basically saying, albeit long-winded, is don't frustrate yourself if you can't find a collector's guide. They're nice to have but in no way guarantees the going value of a particular kit.
Mike