A cruise can be a very economical way to see a lot of different places. Here we will compare the costs and benefits of a cruise vs a trip with a rental car and hotels.
Price:
Cruise: If you get a good deal on a cruise you could pay as little as $50/person/night, an average deal might be $100/person/night. Different types of rooms or ships can increase those numbers dramatically. Add in tips (sometimes a mandatory service charge) of around $10-$14/person/night.
Hotel and Rental cars: This can vary greatly, if you are a bargain hunter the car might be around $15/day and a hotel around $100/night. These numbers can also vary greatly.
Getting around:
Cruise: On a cruise you will go from one city to the next, usually doing the travel while you sleep so you just wake up in a new location. You don't get to experience the countryside in-between locations, but you also don't have to deal with the stress of finding your way around. Getting from one place to the next is included in the price of the cruise. For many destinations you can't even compare the two, since you can't drive between islands, for example.
Hotel and Rental cars: Depending on how far you'll be driving between stops you'll need to add up gas prices or train tickets. You will have a lot more freedom though, to travel around and take side trips.
Food:
Cruise: On a cruise the food is included, so unless you want drinks other than water or you want to eat in extra fancy restaurants on board, the food is included.
Hotel and Rental cars: One cost of a vacation that is often over looked is the cost of eating. Since every meal is at a restaurant this is a cost that adds up fast. Since you know what food you prefer and how much you tend to spend, just remember to factor food as a cost into your trip.