Oh man, this thread is painful to read. It's hard to believe cards were so cheap, especially when they appeared so expensive at the time.
Prices aside, the big regrets for me are generally are the cards I've never seen for sale again. A big part of my collection is now Japanese baseball cards from the 1930s-60s. I can vividly recall at least a dozen cards I thought about buying, didn't pull the trigger, and have never seen for sale again. And I don't even care about the condition!
The production run on some of these cards was very low, as is my estimate of the survival rate. But one hope I hold out is that with increased cards prices and media exposure, caches of these cards will be discovered in old Japanese houses or storerooms or whatever and make it into the market. It might be the one upside I see to the great increase in prices....