I think it really depends on your carrier and the best, BEST answer to this question is to call them and ask all your questions.
I just did a live chat with Sprint and asked them every tiny detail under the sun for our cruise and I was pleasantly surprised with the answers.
For Sprint, you have to activate International roaming, which is free to do.
Then you are only charged for what you actually do. $2.99 per min for calls from the ship (Clearly TOO expensive!!) and $.50 cents a text message to send (only a nickel to receive a text). This is from the ship. The rates in the ports vary, but are comparable.
To me, the texting can be an affordable way for me to just check in with family and be available for emergencies should they need to contact me. If I shut off all those automatic text message notifications I get from Facebook, my bank, different apps, etc. then I can limit my texting to a few to my family over the course of the cruise.
Wifi would be the Disney rates on the ship, and if I can find wifi at the ports, it would be at whatever rate that is (or free with purchase at some locations in the Bahamas, I've heard about).
If I'm connected to wifi on land, and the connection is good, I can Facetime my family just to say Hi. There would be no additional charge for that. So, IMO, Sprint is reasonable. I will probably use it on our upcoming cruise.
Call your service provider and see what they offer and what the rates for you would be and plan accordingly.