(do-it-yourself guide)
Hi,
If it helps, the valuation board reaches most of its estimate recommendations by analyzing market trends over time, as a cost-to-point ratio. (For example, once you break down the Acaeum scale into a 1-10, if a 7-quality sells for $70, this item is very roughly worth $10 per point.)
That raw data is then refined by averaging all existing noted trends, so that the highs and lows and the blurry conditions cancel one another out. To accurately reflect rising and lowering prices, estimates are weighted according to time; 2007 prices, for example, are weighted heavily and 2002 items are weighted very lightly (since their trending is likely obsolete for all but historical purposes).
For particularly obscure items, however, sometimes all you'll get is that one auction with one heavily-contested price, which is when the cost-to-point gives you all you're going to get.
My advice is to grab what you can from eBay auction history and take it as the median. You can go more deeply by searching online vendors, but their prices tend to be high, so maybe cut by a reasonable percentage. That's as close as you're going to get on any item in this field. (I'm answering in detail because this paradigm will prove valuable in collecting not just one particular item, but anything gaming-oriented you might like in the future.)
Caveat: RPG collecting has a heavy emotional component, and a smattering of traditionalism as well - so even with the strength of numbers behind you, expect some howls of indignation if your data, historical gospel and Interested Buyer X are not in agreement. But we won't go into that here.
