I believe all Henry Pym does in TTA #27 is pour the shrinking sermum on his arm, shrink down to the size of an ant, explore, then it wears off and he grows back to normal size. It's TTA #35 where he really becomes the superhero, with his new cybernetic helmet, speaking to and controlling ants, strong as a man but made like an ant, etc. If the splash page didn't say "Return" of the Ant-Man in #35, I don't think there would be much argument here. Proponents of #27 argue that how could Ant-Man "Return" in #35 if he hadn't already appeared in #27.
I get the scarcity angle on the price, but #27 is generally accepted (and labeled as such) as the 1st appearance of Ant-Man...and I don't think that's correct.
http://berkeleyplaceblog.com/2017/01/13/tales-astonish-35-1962-first-appearance-ant-man/
TTA #27