I'm reasonably certain that chain shirts aren't sewn.

I'd agree with Oeryn that it's underpriced, however. For a price of +100 gp, in the Eberron campaign setting, one can have shadows sewn into the weave of one's clothing, giving a +1 hide bonus. Ordinary grey thread wouldn't seem to be nearly as effective... So rather than under-valued, I would say overly effective. Perhaps this fine outfit should instead be restatted as a merely roleplay-worthy item? I would still buy it, if I ever played rogues...