Depends on Brand, Features, and merchant selling its markup. I buy my hardware through
www.newegg.com and a 500watt PSU for this build would be like this one listed for an example of a price tag that I'd pay:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027&Tpk=corsair%20500%20watt%20builder%20series&IsVirtualParent=1This PSU is offered with free shipping, $10 off at time of purchase + $20 off rebate so that in the end you end up with a high end power supply at the bargain of $40. Not sure if free shipping is only within the USA though, but this is an example of a brand and deal that I would take advantage of if I needed a 500 watt PSU.
I like the Corsair Brand, as well as the fact that its rated 4 of 5 eggs (stars) for 360 people leaving feedback responses. And getting this $70 PSU for $40 is a good deal
*Actually finding this deal, i almost want to buy one to keep on hand as a spare. I have 1 system running an old power supply that came out of a Pentium 4 DELL tower, and DELL is one of those hit or miss computer brands for quality amd performance. For example I have seen so many troubled DELL brand computers in my time generally due to component failures, but yet I have a DELL Inspiron 4000 laptop running a Pentium III CPU that wont die