Whether you re buying a $50 MP3 player or a $500 TV, paying more to extend the warranty or service plan is usually -- but not always -- a pretty bad deal.

Buying into such options means you re betting on a rare event: that the product will break during that slim period of time after the standard manufacturer s warranty expires but before the extra coverage does. It s also a gamble that the product will meet its demise in a way that s covered (i.e., mechanical failure as opposed to an inadvertent dunking in the pool).

Of course, that s exactly why retailers push extended service plans and warranties -- that $50 (or more) is 100% profit if you never have to use the policy, says Vipin Jain, the chief executive for Retrevo.com, an online electronics marketplace that offers user manuals and consumer reviews. It s a cash cow, he says.

Even if the stars perfectly align for your gadget to die in an acceptable way during the covered period, technology evolves so quickly that in most cases you could replace the item with a new one of similar or better quality for significantly less money than the cost to repair it. Today s HDTV prices are roughly a fifth of those four years ago, says Alex Goldfayn, a consumer electronics consultant who hosts the nationally syndicated radio show The Technology Tailor Minute, where he helps consumers find the right gadgets to fit their needs. You re going to save more money by buying a new one, he says.

Still, there are some circumstances where extra coverage can be a smart investment. Take a closer look at protection options if one or more of these five characteristics describe the item in question:

Worth $1,000 or more

The pricier an item, the more consideration you should give to paying for coverage, says Michael Carnell, the founder of Charleston, S.C., information technology firm Palmettobug Digital, which helps consumers select and set up technology. A good rule of thumb: Would it be a financial hardship to shell out the purchase price a second time within the next three years, the timeframe of many extended warranties? For example, paying $100 to extend your Dell (DELL) Adamo XPS laptop warranty to three years might be preferable to paying $1,998 for a new one, or even $200 for what would otherwise be an out-of-warranty repair.

Refurbished

Warranty terms on floor models, open-box units and other refurbished items tend to be less generous. Olympus, for example, offers a one-year warranty on most new cameras, digital recorders, binoculars and other products. But it covers refurbished items for just 90 days.

Cutting-edge

Newer technologies tend to be buggy, says Jain. There s no long-term record of reliability, so it's tough to say what problems could pop up and when.

Prone to problems

Read expert and consumer reviews of a product to see if your gadget has a track record of requiring pricey fixes within the extended warranty period. For example, rear-projection TVs are more likely to require repairs than other types of sets, according to a November 2009 Consumer Reports study. The $300 bulb powering the set has an average shelf life of 5,000 hours -- which avid watchers are likely to burn through in a year or two.

Fragile

One fumble is all it takes to wreck a smartphone, digital camera or laptop computer. While manufacturers warranties don t cover accidental damage, a handful of extended warranties and other protection plans do, says Carnell. For example, Sprint (S) Equipment Replacement Program costs $4 a month, plus a $50 deductible, to cover a phone against loss, theft and accidental damage. That s peanuts compared with the $549 replacement cost for a Palm (PALM) Pre if you break it within two years (i.e., the length of time it takes to earn an upgrade discount from the carrier).