Last month I paid substantially more than $400 for a 4" 66 no-dash in more or less new condition that was manufactured back in '72. I may have paid a bit more than market but I don't regret it a bit. The gun is a fine shooter, very accurate, and it balances beautifully in my hand. I've come to believe that K-frames are the epitome of Smith design and the gun assumes more value (not necessarily market, but personal) for me because Smith no longer makes K-frame magnums. Personally, I think that $400 is an excellent price if the gun is in good condition. I'm certain that you'll enjoy shooting it
Smith manufactured 66's for almost 25 years and there were a number of design changes that Smith implemented during that period. The very first 66s, like mine, came with wooden target grips, forged parts, pinned barrels, and recessed cylinders. Some folks consider those features to be pluses and draw invidious comparisons with later models which came with rubber Hogue grips, MIM parts, screwed in barrels, and non-recessed cylinders, and of course, locks. Personally, none of those changes make any difference to me, I'd be just as happy with a newer model than with an old one.
The only change that I find to be significant between newer 66s and the original model is the sights. The very first 66s, such as mine, came with all stainless front and rear sights. After a few months Smith switched to a red insert front sight and, eventually, to a black rear sight. I find the all stainless front sight to be a minor drawback because, in some light it just sort of disappears into the distance. I suppose I could change mine but I won't because I like to keep my gun authentic. Aside from that, I'd say that the 66 is the perfect revolver!