vintage isn't always more expensive. I'll use the 670 as an example. A new one ranges from $2900-$3700 depending on leather used and wood type. A vintage one in good shape will go for $2500-3000. Now, some people like the vintage and there is nothing wrong with that. However, vintage furniture almost always needs some work. A lot of the 670s I see come in need new leather due to severe cracking as well as new foam cushions because they have worn out and become thin. Herman Miller still produces the old style cushions and latches for them for this reason. There is another 400-600 bucks. We'll assume the arms, sides, backs, and gliders on the base are still in tact.

Other pieces, like first productions, limited editions, etc. etc. sell at auction for ludicrous amounts all the time. This is because museums, private collectors, and rich people who buy pieces because they like to tell people how expensive they are buy them. We sold a Bertoia sculpture for over 300k in 2007, a corbu lounge for $45,000, and various other vintage pieces over the years and people buy them. They like the patina that comes with age. A vintage Wegner oak wishbone chair is going to look completely different from a current production Wegner oak wishbone chair.

another note:

even if a design is still in production today, it does not mean it is identical. I could name countless examples of current productions by the original manufacturer that are not even close to being the same.

here's one...since herman miller and Charles/Ray Eames seem to be the theme here:

a vintage eames shell chair is going to be fibreglass in different colors depending on era made. A new one, by herman miller, is injection molded plastic...complete with a parting line and sink marks where the plastic cooled in thicker sections....nasty.