Purchasing insurance directly with Access America (the company Disney uses to underwrite their insurance) is cheaper than buying through Disney, but the coverage is slightly less than Disney.
I bought through Disney on first trip, and directly through Access America for our upcoming trip.
I don't remember the coverage for Disney, but Access America is as follows:
Medical Expense Coverage - 10,000
Medical Transportation - 20,000 (I think Disney has 30,000)
Baggage Insurance - 750
Baggage Delay - 200
Travel Delay - 500
Missed Connection - 500
Buying more Blue Drinks on board with the $ I saved - Priceless
The cost is dependent on the price you paid for cruise. Go to their website to get quotes.
The one thing to remember - to get the prior condition waiver, you must purchase the insurance within 14 days of paying your deposit for Access America, its 30 days for Disney.