Here is a fun study from 2012, before CAC really started to take off courtesy of billionaire-driven bidding wars at auctions. It found that "CAC-stickered coins are selling for higher prices....No exceptions were found for any series or grade studied."
coinweek.com/educational/coin-grading/comprehensive-market-study-reveals-cac-price-premiums
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2. Most extremely high APR's resulted from said bidding wars, such as the $364,250 APR for the painfully common 1938-S CAC dime versus its then-$4,750 CU in the same grade without the PCGS "+".
3. I suspect that said outrageously high APR's have inflated the values of all CAC coins, even for inexpensive ones (under $500,000) in which we lowly mortal dealers deal. All of us cannot be a "Legend" (pun intended).
Your premise is foolish. In the current market, quoting occurrences of "Black Swan" events and characterizing them as the market levels that they most surely are not is misleading.
Is there an updraft effect? Is the 1938-S 10 PCGS MS68+ FB CAC APR relevant to any other 1938-S dime. To any other Mercury Dime? What market level is that?
You've made a huge deal of your putative eBay experience. Regrettably, that's about the last venue many would suppose is a healthy market for CAC product.
I suggest that outrageously high APRs may have distorted values at the very high end of a series or for very rare coins within it, but few others.
I'd be happy to give you current examples of premiums of more general relevance, like sandblast proof gold (typical basal/premium divergence) or the more relevant MS65-66-67 Saints. I'm not sure the doubling of many hundreds of thousands of common unc Morgans is attributable to the "Weitzman Effect"
Yes, I am "foolish" yet you know everything because you are an ex-TPG grader. Got it!
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Point of fact. The 1938-S 10c PCGS MS68+ CAC referenced above by the OP was not, as stated erroneously, upgraded from a 68FB coin, but rather from an MS67+FB CAC