Booking a cruise? Do some math first.

The cruise industry bills cruising as a great value. For one upfront price, your accommodations, meals and trip are paid for.

But most cruisers end up spending more. Carolyn Spencer Brown, editor of CruiseCritic.com, says her rule of thumb is to assume she’ll spend half the amount of the cruise fare on incidentals. She recommends an online cruise budget calculator to help estimate all your costs at http://www.independenttraveler.com/travel-budget-calculator/.

Wendy Perrin, consumer news editor at Conde Nast Traveler magazine, also cautions that “the cruise fare you see advertised is not the price you’ll end up paying. The advertised fare, which is usually for the smallest, windowless cabin, does not include mandatory taxes and port fees, mandatory gratuities, or highly recommended additional charges, such as cruise insurance.” Perrin blogs about travel at http://perrinpost.com.

You’ll also have to buy plane tickets to the departure port if it’s not within driving distance. But there are ways to keep costs down. Here are a few:

CABIN SELECTION: The cheapest cabins will be inside—no balcony, no windows. This might sound depressing, but on many cruise ships, public spaces are so vast and appealing that you won’t want to be in you’re room unless you’re sleeping.

There are multiple decks with pools, atriums and libraries—all to be enjoyed for free—not to mention the scenic views. On an Alaska trip, you’ll spend part of a day at sea cruising past an enormous glacier—an incredible sight, no extra charge.
FREE ACTIVITIES: Plenty of things to do on a cruise ship are free, like using the fitness center or catching live performances, from comedy to musical variety.

The Celebrity Solstice, which launched in November, features the first “Hot Glass Show” at sea, where glassblowers from New York’s Corning Museum of Glass give free glassblowing demonstrations. The Solstice also has a deck with real grass called the “Lawn Club” where you can play bocce or croquet or putt golf balls for free.

On Princess Cruises, one way to lure you out of your room is a giant poolside screen, 300 square feet, that shows feature films (including first-run movies and family shows), concerts and even telecasts of events, from the inauguration to the Super Bowl to the Oscars. Princess calls the program “Movies Under the Stars” but there are broadcasts all day as well as at night. The screens are now on Caribbean Princess, Crown Princess, Emerald Princess, Ruby Princess, Grand Princess, Star Princess and Sea Princess, and they’ll be installed in 2009 on Golden Princess, Dawn Princess and Coral Princess.

Heidi Allison-Shane of CruiseCompete.com notes that on many lines, if you book a suite, you get additional freebies, for example complimentary passes to thermal lounges in spas.

AVOID TEMPTATION: If you’re counting pennies, skip the casino, spa, specialty restaurants that charge extra, the gift shop, art auction, fitness classes, lattes in the cafe and cocktails at the bar.

But don’t underestimate how hard it is to resist the repeated announcements and flyers promoting these things and others. Additional pressure comes from hearing other passengers rave about massages, yoga class, or in-port helicopter tours.

“You kind of feel like maybe you’re missing out if you don’t try this or that,” Perrin said.

One compromise: Give yourself a daily budget for extras, and if you skip a day, you’ll have that much more to spend the next day.

KIDS STUFF: A huge plus for families: Most cruises offer free children’s programs and kids usually enjoy them. But children like to spend money on cruises too—in the arcade room, for example; on milkshakes and other goodies which may be extra; and for activities like rock climbing.

Give kids a daily allowance so you don’t have to negotiate or be constantly nagged—$5, or if you’re feeling really generous, $10 a day. If they’re hoping for something in the gift shop, they can save up over a few days.

Do buy the wristband or card that covers unlimited soft drinks. It pays for itself in no time. You’ll want one for yourself too.

SIGHTSEEING: Do your homework about different types of cruises. For fun in the sun, cruise to a beach destination or warm island. A cruise can be a “wonderful way of island-hopping,” Perrin said.

But to explore a destination in depth, a cruise may not leave you feeling satisfied, and could entail hundreds of dollars extra for ground transportation and tours, Perrin said.

“You might have eight hours in port or 11 hours in port, but depending on the destination, it could take hours to get from the port to the city,” leaving much less time for sightseeing, Perrin said. For example, on a Mediterranean cruise, getting from the port nearest Rome to the city center and back can take four hours.

Shore excursions booked through cruise lines are also usually priced higher than if you made arrangements yourself. “You can often have a better time for less money hiring a car and driver,” Perrin said.

But there are advantages to booking excursions through the ship. Cruise lines screen tour companies for quality, and if there’s a delay in ground transportation, the ship will wait for you if you’ve booked the tour through them.