It all depends on the brand name and quality of parts. For example my Antec Sonata II came with a special SmartPower2.0S Powersupply by Antec. Antec makes solid powersupplies so I saved myself a bit of money not having to buy another PSU.
If you're going to get a cheap case, then invest in a good power supply from a maker like Antec, OCZ, or Fortron FSP.
But if you're getting a cheap case that comes with a no-name or cheap powersupply. Then ditch it, its not good for you and will kill your components.