I'm often surprised at the variance in prices based on geography, seller and circumstance. Most especially, I'm consistently puzzled by some people's seemingly quite low assessment of what's a reasonable, market-based price for a given firearm.
$400 for a Model 13 that's in very good or better cosmetic shape and (most importantly) mechanical shape is a great price by my research.
The 4" heavy barrel Model 13s, while not rare -- like, say, its 3" round butt brother -- aren't particularly common, either.
Older Smith & Wessons in good shooting shape are becoming rarer and increasingly sought after; even the prices that applied a year ago don't seem to stand now, and not everyone appears to be following the trends or, if they are, are willing to accept them.
To the OP: if the Model 13 you're looking at is excellent to very good (by NRA modern condition standards) cosmetically, timing, function and specs-wise, $400 is a steal. If it also happens to be the gun that feels right in your hand and will suit your needs, buy it.