Whether VoIP (such as Skype) will work acceptably or not is very dependent on how many people are using the satellite internet at the time. Sometimes it work wonderfully, and at other times, it's too rough to carry on a conversation.
You can usually estimate VoIP usability by making an educated guess as to what the average guest is doing. For example, afternoons on sea days during the weekday are especially rough, especially toward the end of a cruise, as that's when a great many people are using the Internet to get caught up on email and other work activities. VoIP performance late at night, or during show/dinner times, is much better.
As to the cost, I like to look at it on an hourly basis: $18/hour for WiFi (assuming you buy the largest package), or $150/hour for cellular roaming (assuming your rate is $2.50/minute). So, I think it's always wise to try VoIP over WiFi first.