Why do 'thin' computers cost more than PCs?

Something that has always struck me as odd is the fact that thin computers cost more than full fledged computers even though they're not nearly as well equipped with things like high-performance processors or local hard drive storage.  Take the brand new Sun Ray 2FS for example which starts at $499 with no monitor or the Sun Ray 170 comes with an integrated 17 inch LCD display that runs $869.  Either product cost way more than a full fledged PC like the Dell Dimension 1100 which includes a 17 inch LCD display which can be purchased as low as $349 or similar deals with 19 inch LCD displays for only $50 more.

Now to be fair to Sun, other thin client manufacturers like Wise are fairly expensive too, but am I the only one that sees a problem with the entire thin computing pricing model here?  Sure you have some impressive power saving features where the Sun Ray can operate with less than 10 watts of power (excluding monitor), but low voltage is easy to achieve when you don't have a hard drive and you don't need a lot of local CPU performance.  The problem is that it's going to be difficult to justify the initial expense when the cost difference is so high even though you can make it up in a few years in electricity bills.  With newer low-power processors coming out at the end of this year, low power computing will be more common in value PCs and the power difference will be slashed to 20 watts or less per station which will make the electric savings even more difficult to justify the additional expense of thin computers.