Although the Cat 8D is "cheaper" than the Cat 10, it was not that way on opening day. On that day, the inside staterooms were less expensive but they booked up quickly causing Disney to raise their prices more rapidly than they have with the outside/verandah staterooms.
Look at this thread:
http://www.disboards.com/showthread.php?t=2579000
2A2C were able to book a 10A for around $3,500 with the 10% discount. Without the discount, that room is about $3,900 on opening day with 2A2C.
Right now, that same 10A room is going for $4,570, an increase of $670 from opening day.
The Cat 8D for 2A1C was going for around $3,441. That same room for 2A1C is around $3,553 so it's gone up about $112 since opening day.
This means that more people booked up the less expensive inside staterooms immediately after opening day. If you want the lowest prices possible, it's often best to book them on the day bookings open. Otherwise, you could end up paying more - sometimes a lot more!
We booked two staterooms (1 Verandah and 1 inside) on the first day for the 6/2/2012 cruise and the inside room has gone up by $530 whereas the verandah room price has remained unchanged.