The thing to know about DSLR's is the ISO (not the megapixals, like most people think). The cameras I listed above are about 12-15 mpix. Most of the top cameras are just in the low 20's, so 15 mpix is not that bad. Max ISO on the cameras above are 3200, making it difficult to shoot in low light with out a flash. My camera (Canon 60D) has an ISO of 6400. I can grab some low light shots, but not like the new cameras that have ISO's over 20K (up to 100K). These things can produce some amazing images without noise in low light due to the ISO.

The Canon 60D (body only) can be purchased new for under $600. The higher ISO cameras can get up in price quickly.

The higher ISO is not the only reason for the higher prices, there is new processors, etc. Just like the world of computers changes and improves by leaps and bounds day-to-day, so does the DSLR camera world.