I'm also a new MSC cruiser and also wanted to let you know that unlike some of the other lines, it seems MSC does not tell you the times of your excursions until on board (and apparently often the night before, per the answers I received on this board when I just asked about it). Now that may not matter to you if that's all you planned to do for the day, but I did find it a little frustrating. That said, the prices were pretty unbeatable so I can't be too mad about it.
Example of pricing based on my upcoming cruise to Northern Europe. Please note sometimes NCL will offer $50 off per stateroom per excursion, so for you and your wife you could potentially subtract $25 per person from their numbers. MSC is pretty much always cheaper, even with that $25, sometimes significantly so. I'm not sure if Carnival offers deals as I haven't cruised with them in almost 20 years. Sorry that I can't help you with pricing for the Caribbean specifically.
Le Havre, France:
Rouen is $71 on MSC (5 hours) and $119 on NCL (5.25 hours), no option for just Rouen on Carnival
Honfleur is $83 on MSC (4.5 hours), $109 on NCL (4.25 hours), and $90 on Carnival (4 hours)
Le Havre by foot is $41 on MSC (3 hours) and $75 on NCL (3.25 hours)
Paris Sightseeing tour is $126 on MSC (10 hours), $199 on NCL 10.25 hours, and is $200 on Carnival (10.5 hours)
Mont St Michel is $126 on MSC (10 hours), NCL does not offer it, and $180 on Carnival (10.5 hours)
Zeebrugge:
Bruges Walking tour is $46 on MSC (4.5 hours) and $89 on NCL (4.25 hours), Carnival's tours are longer (8 hours) and are $110 for a walking tour and $79 for on your own.
Brussels is $69 on MSC (8 hours) and $109 on NCL (8 hours), Carnival's includes more of a guided tour and a lunch and is $130 (8 hours)
Ghent is $52 on MSC (5 hours) and is $109 on NCL (5.25 hours), Carnival's incudes the canal boat tour ($11 to do on your own) and is $120 (5 hours)