its because the 400's were specific to some completes i.e the laserlite which makes them more in demand as if you are building a laserlite you need 400's which makes them desirable pushing the price up.
Wouldn't that only be the case for Sugino 400's
I believe it is only down to the fact there are less around due to 401's being the superior choice BITD.
It would seem there's quite a few things that fetch high prices due to rarity and not coz they're better.