Alongside iPads, e-readers and Pillow Pets, this year s letters to Santa have an unexpected entrant: the humble sweater. But although advertised sweater sales are plentiful with prices sometimes as low as $20 truly good deals are few and far between.

Even though $25 sweaters of all kinds can now be found at almost every retailer, it s tough to tell what s worth even that rock-bottom price, says Asta Skocir, a knitwear design consultant and an assistant professor at the Fashion Institute of Technology in New York. Some retailers are able keep their costs low and to offer customers lower prices by making sweaters in bulk. Others get to that $20 tag partly by using lower-quality fibers or by cutting corners on production. And the colder it gets, the more willing customers are to throw a cheap sweater in their cart. It s the consumer psyche when you re cold in line to check out, sweaters get added to the list, says Scott Bernhardt, the chief operating officer for Planalytics, a consulting firm that specializes in analyzing weather-driven demand.

A temperate October and early November prompted retailers to cut prices on sweaters, which is one reason sweaters were one of the apparel industry s strongest categories last month, analysts say. They also note that the American consumer might be due: The last few years haven t been sweater years, says John D. Morris, a senior analyst covering retail for BMO Capital Markets. "There's pent-up demand." Now their popularity could eclipse even that of gift cards this holiday season. Teens are following the trend too: When the firm recently gave teens $100 to spend at the mall as part of a study of teen buying habits, sweaters were the No. 1 item they picked, he says.

The low prices could, in some cases, lead to good buys. An inexpensive sweater can be made well enough to last you more than a season, Skocir says, if it s a good blend and a thick knit. Thinner knits -- where you can see your hand through one layer -- wear out faster, especially around the elbows. As for which blends are a good buy at such a low price point, cotton and wool are still shoppers best bets. But they might be harder to find: Designers are using fewer natural fibers as prices rise, adding in more man-made ones such as acrylic or nylon. In small quantities, those add stretch, but in higher quantities they are also more prone to pilling (those fuzzy balls that collect on the surface and make a sweater look old and worn), Skocir says.

Some of the current sweater deals that qualify as legitimately good buys include Lands End cotton baby cable V-necks (currently half price at $30), Old Navy rib-knit crew-necks ($25, down from $30) and Victoria s Secret silk and cashmere V-necks ($29 instead of $39). (Hunting for an all-cashmere sweater? Those more expensive garments have their own requirements

If you ve already been drawn into a cheap sweater deal, or if you've received one as a holiday gift, follow its care instructions to get the most value out of the purchase. Generally, cold water and low-agitation (delicate) settings on the washer are best, says Lorraine Muir, the director of textile testing services for the Drycleaning & Laundry Institute, an industry group. Lay the sweater flat to dry heat makes fibers like wool shrink, while acrylic will stretch. Or, for $25, you could just buy a new one.