Disney does have promotions but they will never be in line as far as pricing with Carnival. Typically Disney is about double the cost of CCL. Your best bet is to try to find one of the mass market .com cruise providers that has purchased a large block of cabins with Disney. That is the only time you'll ever see Disney pricing competitive with the other mainstream lines.
The name Disney and having Snow White onboard just seems to cost mare and people are willing to pay the difference.