Well, I realize they are profitable, but since AT&T is a partner with MTN in providing the service of Cellular At Sea, they don't need to be outrageous, which I believe they are currently.
As an example, Verizon has cruise ship service at $25 per 100 Mb with no per minute rate. The AT&T plan you quoted as $120 for the same 100 Mb seems ridiculous in comparison. I'm pretty sure Verizon isn't losing any money on this rate.
For the first half of this year a company called "KeepGo" had a rate of less than $10 per day with a 100 Mb per day limit for cruise ships. I had used this many times, but as of June, it was discontinued. The old AT&T Global Data plans also still include ship board data.
There is also a new service being used in the Caribbean which provides 3G service on ships via low orbit satellites that is available of selected ships. Hopefully this competition will cause the carriers to rethink their charges.
T-Mobile currently provides free data roaming around the world, and $.20 per minute voice service also. This leads me to believe the cost to carriers for their roaming rates are no where near what they have been charging us for this feature.