sometimes hard to read on the phone and want to type out good responses, but I got to thinking about this...it is not that I won't admit it, it is that I believe this statement to be false...
in fact, the only example off the top of my head (keep in mind, talking about comparable grades at comparable times) I can find of a 29 selling for less than a 31 was the dark knight 4.5 that sold for considerably less than his 4.0...
the 4.0 was visibly a superior copy and the 29 was in some folks opinion, not near as visibly nice for the grade... but outside of that, I can't think of a single other example to support your statement (though I have to admit I might be missing some, but I can't recall them)
even going back a few years, I paid more for my 29 cgc 3.5 than I did for the 4.0 tec 31 I bought at the same time...I already gave the other example of the 2.0 #29 selling for more than a 31 in 2.5 (pq and all)...
when the 29 cgc 5.0 sold for 25K on CC, heritage sold the 5.5 31 for just 10% more (comparable)...
I have pretty much always sold copies at comparable rates (I have sold 25 tec 31's and 12 tec 29's, some the same book mulitple times)... so I would say apples to apples, tec 29 in most cases (not all) will sell for the same or :gulp: even more than a 31
but of course, the market changes fast sometimes, so your milage with vary