Well, the issue is that you're going to want a place that you can return it even if the thing has been opened and used.
If you can shop online (the cheapest way to go, but no returns), you can get a brand name (Thermaltake) 420w for $43 here:
http://www.ewiz.com/detail.php?p=PS-W0009R&c=pw
Or a generic one for much cheaper ($12) here:
http://3btech.net/viopo42posur.html (could be just as good for all I know).
Otherwise, if you're looking in a store, I think you're probably looking at $60 (give or take 10 bucks).