having built over 1.5k pc's I would have to say its not just cheaper...
its better... prebuilt rigs are very constrictive and often have the cheapest value parts... [where possible]
specially off the shelf like in a high street mega retailer like PCWorld or Currys or Wall Mart or whatever wherever you live...
they come loaded with gunk that will hinder your experience and true power potential... also having built it yourself brings about total comfort, as well as fear of failure, but the achievement of building it brings about motivation and shear love and care for your baby I mean computer...
you should see my rig... she is a beaut! [£1k+]