That's the thing... I am unsure they actually are that valuable. I think it may be like Nebraska Kirk said - just the result of a bidding war.
There is an old IHC 50/51 UD (I think) on ebay for $99. I bought one exactly like it for $18 and have seen them sell for as little as $10. For that reason, I don't trust ebay as a price guide unless I have watched many of the same item sell - so you can catch an "average" of prices.