I'm not sure why the big difference, but I imagine that it has a lot to do with the fact that on a point and shoot, the sensor does double duty as both a view finder and a capture sensor. With a DSLR, the view finder is a totally different system from the sensor.
The other thing to keep in mind is that even the base model DSLR's like the D40x, are more expensive than 95% of the digital point and shoots. Again, you pay for performance.
David Buzzard's Technical Blog