There are quite a few S&W revolvers that can be purchased as "previously owned" guns for under $400. A used Model 36 "Chief's Special" in .38 Special will run anywhere from $295 to $395, depending on condition, age and the local market of your area.
When looking at used revolvers, it is the mechanical condition you should pay attention to first. Clean, bright bore, little or no flame cutting on the topstrap, firm but smooth trigger and good timing. The appearance of the finish is a secondary concern. As long as the metal isn't heavily rusted or pitted and doesn't look like it was used as a hammer, a worn finish can be corrected at a later date.
If weight means a lot to you in a CCW gun, the Airweight models will probably make you happier than a steel framed gun. Although, if you plan to shoot your carry gun at all frequently, I recommend a steel framed gun for durability as well as recoil reduction.
The S&W 642 is an excellent gun, but I prefer the Model 38 "Bodyguard Airweight" with the humpback hammer shroud. The lightweight guns will cost more, but that is offset by the comfort they offer for all-day/everyday carry.