Every week, it seems like another airline or hotel is touting buying points or miles. We've recently seen promotions from a 45% discount on buying United Airlines MileagePlus miles to a whopping 145% bonus when purchasing Avianca Airlines LifeMiles. But is buying frequent flyer miles a good deal? That depends on a number of factors and comes down to how you use your points and miles.

Let's dig into when it can make sense to buy points and miles and when you should avoid it.

Is buying frequent flyer miles a good deal?

And is buying miles worth it? It's rarely worth it to buy frequent flyer miles — especially without a bonus or discount. Airline and hotel programs generally price points and miles at such a high rate that it's hard to get more value than what you paid for them.

Let's take Alaska Airlines Mileage Plan miles as an example. Alaska generally sells miles at a base rate of 2.75 cents per mile. After adding taxes and fees, your all-in cost is around 3 cents per mile.

NerdWallet values Alaska miles at 1.4 cents each. So it doesn't make sense to buy miles at almost 3 cents each if you can expect to get around 1.4 cents of value.

When should you avoid buying miles?

When you don't have a specific use for them

Airlines and hotels can devalue their loyalty programs at any time. So be cautious about buying points and miles without a concrete plan to use them. What might be a good deal at the time can end up not being as good of a deal if the airline were to change its redemption rates.

When you wouldn't pay the cash price

Just because you're getting an incredible redemption rate doesn't mean you're coming out ahead. For example, you can redeem 50,000 Alaska Mileage Plan miles to fly Cathay Pacific business class to Asia. We found a Cathay Pacific business class flight between the U.S. and Hong Kong that cost $8,464 round-trip, meaning you'd get a redemption rate of 8.5 cents per mile for this flight.

But how much would you actually pay for this experience? Before you spend anywhere from $2,000 to $3,000 to buy 100,000 Alaska miles to book a roundtrip Cathay Pacific business class flight, make sure that you're happy paying that much for the flight.

When you're short on cash

Avoid buying points and miles if you aren't able to pay off your credit card bill in full. Most rewards credit cards charge high interest rates. So your mileage purchase will end up being a lot more expensive than the initial price if you have to pay interest on the purchase.

When might it make sense to buy miles?

While it generally doesn't make sense to buy points and miles, that's not a hard-and-fast rule. There are some rare situations where you can actually get more value for points and miles than what you paid for them.

A lucrative promotion

Let's circle back to the Alaska miles example. While the standard price for Alaska Mileage Plan miles is almost 3 cents each, Alaska often runs miles-buying sales. By taking advantage of one of these promotions and the sweet spots of Alaska award flight redemptions, it could make sense to buy Alaska miles.

Recently, Alaska featured a promotion that offered 60% bonus miles when buying at least 40,000 miles. That means that a 40,000-mile purchase would have netted 64,000 Alaska miles after the bonus. Factoring in this mileage bonus, the effective purchase price of Alaska miles dropped to 1.85 cents per mile after fees.

At that rate, it can make sense to buy Alaska miles for certain redemptions. For example, Alaska miles tend to increase in value increases when booking Alaska business class award tickets. If you're able to buy miles at 1.85 cents each, then redeem them for 2.8 cents each, you're coming out ahead.

The value proposition can be even better on Alaska partners. For example, you can book awards on Fiji Airways from the U.S. to Australia or New Zealand for 40,000 Alaska miles in economy or 55,000 Alaska miles in business class each way. Those are some solid redemption rates.

Plus, by booking with Alaska Mileage Plan miles you can also take advantage of Alaska's free stopover policy. That means you can visit Fiji for as long as you want before continuing on to Australia or New Zealand. For an incredible redemption like this, it can be much cheaper to buy and redeem miles than buying a cash flight.

For the chance to take advantage of incredible sweet spots

Some sweet spots are so good that it can justify buying points and miles even without a discount or bonus. For example, Etihad Airlines' loyalty program Etihad Guest offers some of the best sweet spots in the points and miles world. Add in a reasonable mileage purchase base rate of 2 cents per mile and it can sometimes make sense to buy Etihad Guest miles even without a promotion.

Here's an idea of how much you can pay for trips from the U.S. (before taxes and fees) when buying Etihad Guest miles at 2 cents each:

Royal Air Maroc business class to Casablanca for $880 (44,000 Etihad Guest miles) each way.

Brussels Airlines business class to Brussels for $880 (44,000 Etihad Guest miles) each way.

American Airlines business class to Europe, Japan or southern South America for $1,000 (50,000 Etihad Guest miles) each way.

Fly ANA business class to Tokyo from $1,080 (starting at 54,000 Etihad Guest miles) each way.

Asiana Airlines business class to Seoul from $1,180 (starting at 59,000 Etihad Guest miles) each way.

Just make sure to factor in the taxes and fees needed to book the award when considering if it makes sense to buy miles.

Account top offs

Another time it can make sense to buy miles is when you're just short of having enough miles to book a redemption. Sometimes buying just a few thousand miles can increase the value of your existing miles.

Say you have 117,000 United miles. That's more than enough to fly roundtrip to Europe in economy on most dates. However, if you had just 3,000 more United miles, you could book a roundtrip award in United Polaris business class.

In this case, it could be worth it to pay $113 (3.5 cents per mile plus taxes) to buy 3,000 United miles to unlock this business class redemption.

Before buying travel points, see if you can transfer points from a transferable points currency. For example, Chase Ultimate Rewards® transfer to United at a 1:1 transfer ratio. If you have Chase points, you could transfer 3,000 Chase points to United to book this redemption without needing to buy miles.

If you're considering buying travel points

Does it make sense to buy points? Not usually. By default, you should avoid buying points and miles. It's always better to earn miles for free through credit card sign-up bonuses, take advantage of promotions and earn through partners.

However, sometimes it can make sense to buy points or miles. For example, if you can spend less to buy miles and redeem them for a flight than the cost you'd pay for that same flight, you'd come out ahead by buying miles. Just make sure to factor in the taxes and fees on the award ticket and the mileage earnings you're giving up by booking an award flight instead of a revenue flight.