Pricing information is almost impossible to find. Here is what Princess says on its website, and AIRC the information provided at sea was similar:
Neither MCP nor Princess set retail pricing. Retail pricing is set by your cellular provider, and while we would like to be able to provide guidance, it is impossible for MCP or Princess to comment on independent rate plans. When in doubt please contact your cellular provider.
or in other words: We're not going to tell you what it costs!
I know when we used our cell phone at international roaming rates ashore to check with a guide, the rate was $6 a minute! Presumably at sea it would have been the same.
Our solution is NOT to use cell phones at sea!