I think the thread: "
Price Check: How much is my _____ worth?" could be used for that purpose, to a degree. It was made for the opposite question though.
Prices are sometimes discussed in the
Great/Interesting Finds thread also.
One thing I do to estimate something's value is to search on eBay for
completed auctions, where the item sold. Check at multiple auctions and gauge a general ballpark figure, disregarding outliers.
Be also aware that the condition something is in matters also. Sometimes a seller offers very little information about the internal condition or what particular variation/type something is, and then I think the price should be lower.
You sometimes see sellers who ask for too much because they misidentified the condition or variation of what they have, after having seen something better sell for that.
Then there are those sellers who "go fishing": initially asking for a too high price and progressively lowering it, hoping that someone would "bite".
The price level could also be different in different regions. For instance, Cherry G80-series are often cheaper in Germany (where they are more common) than elsewhere. IBM Model Ms sell for much more here in Sweden than in the US, etc.
And as with any collectables, the price
will go up and down as trends change. When I started ten years ago, I could get a good vintage keyboard for scraps, and that time is over. Nowadays, some modern keyboards are too common on the second-hand market that their price is a fraction of what they had cost new.