When you decide to redeem miles or points for your next vacation, do some checking before you cash them in. When prices are high for your trip, it can be well worth using miles or points, but if you can get a cheap trip, save your miles and points for another time.

I recently booked a trip to South America, flying into Buenos Aires and returning from Santiago, and the prices for the flights were so outrageous that I decided to cash in miles. Coach-class fares would have cost $1,040, and business-first class was $7,091. Usually, I will pay for a ticket and use my miles to upgrade, but the prices were too high to make that worthwhile, because I would have had to use 50,000 air miles plus pay $700 round trip to upgrade from discounted coach class.

I have air miles on several carriers, so I like to see if there are any promotions to lower the amount of miles I need to use. I found a mile-redemption deal on American that allows me to fly first class, with a free U.S. stopover; so it cost me 112,000 miles instead of 150,000 miles to fly in first class on the way there and business on the way back.

I will fly to New York and stop over there to pick up my son, who will go with me. We'll then fly on to Buenos Aires, then home from Santiago.

You really have to look at the cost of your miles before you redeem them. I thought about using points for my hotel stay in Argentina, but when I calculated the cost, I decided to pay cash.

I was able to get a five-star hotel for $600 for four nights, including taxes, fees and breakfast. I would have had to redeem 100,000 points for that, and it didn't seem like a good value for my points. I did cash in points for my Santiago stay because it would have cost $1,600 for six nights, so that price made it worth redeeming 125,000 Marriott Reward points.

I plan to go to Europe in the coming year, where the four-star hotels cost $300 to $400 per night. What cost $600 for four nights in Argentina would cost $1,200 to $1,600 in Europe, making it a better value to use my points in Europe.

When you look for hotel rooms, especially in Europe, try to book where breakfast is included in your room price. I've seen breakfast cost as much as $50 per person at four- and five-star hotels in Europe.

You should also consider the cost of packages before you redeem miles or points. You sometimes can get your air and hotel bundled for little more than what you would pay for hotel or airfare alone.

For example, if you are traveling to Mexico and want to redeem miles for your airfare but your hotel alone will cost $1,000 for five nights, you should definitely look at packages. If you can find a package for two for $1,120, you'd save only $60 per person by using miles for your airfare, and that would be a waste of miles.

Tom Parsons is publisher of BestFares.com.