Hmmm, difficult question to answer. Specifically for share purchase, it depends on the rationale for purchase, as briefly alluded to by Romp. I do stuff the old school way, so this concept has never been a significant factor other than for a quick calculation of cost base. If something is cheap at $1, it is even cheaper at $0.50 cents, assuming no factors have affected valuation in the interim.
Happy to talk further by PM.