The best way to see the value is to look at eBay's completed auctions and order them by final bid value - the games that have sold for the highest prices are generally the rarest. Things like Radiant Silvergun on Sega Saturn (regularly sells for over $200, Panzer Dragoon Saga also on Sega Saturn, Wild Gunman on NES (one just sold for $550).
Wild Gunman?
The highest buynow at the moment seems to be $14.95... I've never heard of its rarity before. I sold one copy not too long ago for $4. And I've never seen a copy go for more than (here in Sweden) like $8. Not always complete, and in mint condition, but anyways.
But, in all honesty, eBay (and other online auction sites) are really only good to look at if one bothers about one can get for a specific item. It is only one isolated part of the entire market, and I would never consider paying more than a couple of dollars (if even that) for a game like Wild Gunman (if I only bother about the cartrigde and not the box and manual).
In the case of Radiant Silvergun, the game IS very rare and it is really hard to obtain by other means than eBay, in those cases eBay is pretty much the market for the game and the price is rather valid. Even if one would go to Japan and search all the stores there the price would be REALLY high, and the game would also be a rather scarce find. That is not the case with Azel (Panzer Dragoon Saga) which would be something like €40 in Japan. I also think that Panzer Dragoon Saga were released in higher quantities in Europe than in the US (I might be wrong here), so it would thus be cheaper to buy the European version and run it on a modded Saturn in the US.
Blizzard 1230's sells on some auction sites in Sweden for, like, €150 (often by people that have bought the card on eBay or other abroad auction sites just to make some quick cash). Which is insane pricing (according to me). I would say that what I find a reasonable price for a Blizzard 1230 would be something like €50 - €70. Buying from other sources than eBay (and the like) most certainly makes the price one has to pay drop rather drastically.
I've even seen an external diskdrive for the Amiga go for something like €150 on eBay once. What are people thinking, really?
I sometimes really hate online auctions, and the sites they are hosted on. But, in most cases, I love them.