the faster proc also has more cache.
and as said, a more BA graphics card, and larger HDD!
The 2.5 has s bigger HD and a different graphics card.
I'll answer the second question. Apple loves money.
P.S. The difference between the two models in terms of raw power is less than 10%, you decide whether that's worth over 20% more money.
The 2.5GHz* model has a 250GB hard drive rather than 200GB, and 512MB of RAM on the graphics card rather than 256MB.
*Having the 2.5GHz model also means you're cooler than people with 2.4GHz.