That’s an interesting rule of thumb — I wonder where the “good used = half retail” idea comes from? I’ve always thought, as alluded to, that there are many other factors at play that determine price: market demand, age of item, inflation, scarcity (related to sizes, production runs, place of manufacture), how much of a cult following something has, even regional interest, etc. Also, the metrics for "good used" are highly subjective and variable. With bikes like the Betty Foy/Yves Gomez — long-discontinued, a bit of a Rivendell legend for being the second Rivendell mixte model after the fabulous Wilbury/Glorius, and arguably more “golden age Riv” and desirable than current models
especially for those who prefer shorter wheelbases for any number of reasons — the value can really float above simple formulas, depending on who’s looking.
So while half retail might work as a rough starting point, the real-world market (especially for niche bikes like these and scarce components) tends to be a lot less predictable. And that's before factoring in current new bike prices, which are affected by ongoing political and economic policies.
Jim