When talking about used watches, prices are probably most consistent on Chrono24, especially if the brand and model in question are very well-known and there are many units for sale. There is sometimes quite a wide price range for some (of the same brand and model), but for used ones, condition/service history/full set factors naturally matter a lot. On Tori, on the other hand, it feels like people ask for whatever comes to mind, especially for Rolexes. Also, watches assembled from parts are sold as originals (see Frankenwatch).
Like @Bolse, I wouldn’t buy a watch as an investment either, but for use; I wouldn’t be able to resist using a fine micromechanical object (and the value drops when used, if/as patina develops). Watches are a nice hobby in the same way as stock picking, so why not pay a bit for it.
Value is likely pretty much the same as with gold: whatever a buyer is willing to pay for it at any given time. Of course, “quality watches” like Rolex, Omega, etc., certainly hold their value better. It’s then a bit down to luck whether the value (of a certain model) rises a lot over time, a whole lot, or something else. Rolex seems to know how to manipulate prices; i.e., a good brand and certain scarcity in production keeps raising prices constantly (for new watches).
Then there are all sorts of limited editions, whose prices can jump drastically due to some hype. Overall, demand creates value in these things as well.
Let’s add the FB group “kelloflippaajat” (watch flippers) to @Bolse’s list, where decent pieces change hands every single day. I’d say you can’t go far wrong with a Speedy or a Rollie if you want a high-quality timepiece.