You snooze, you lose. At least when getting cash or credit for your old cellphone.

Waiting even a month before trading in your old phone after a new version is released can mean a loss of 20 percent, said Nik Raman, chief operating officer at uSell, an aggregator of selling used electronics.

As Apple unveils its latest iPhone on Tuesday, rumored to offer fingerprint authentication, Raman estimates that old iPhones will lose 5 percent of their value within a week, 12 percent within two weeks and about 20 percent within three to four weeks.

"You can save more than $70 by timing the sale of your used iPhone," he said.

An iPhone 5, for example, is expected to lose about $72 by the end of the month. The 4s will be worth about $46 less and an iPhone 4 will lose $29 in about a month, Raman said.

Speculation is rampant about the features the new phone will offer.

Several tech blogs, including Apple Insider, are posting a picture of a supposedly leaked iPhone "5S" box with a home button that has a metal ring around it. Others think that Apple's recent acquisition of the fingerprint recognition technology firm AuthenTec and clues embedded in early developer builds of iOS 7 suggest the possibility of a fingerprint sensor.

Several reports, including those critical of labor practices in Apple's supply chain, have indicated that the company may announce a cheaper version of the iPhone that could come in many colors. Apple's media invite — with its tagline "This should brighten everyone's day" and several colorful circles — has some tech blogs speculating that the firm is planning a candy-colored line of less-expensive phones.

With the approach of a new iPhone, values of existing phones have already fallen at Gazelle.com. In a price check Monday, a 16GB iPhone 4s in good condition was worth between $150 and $170 cash at several sites. The trade-in value for store credit was $150 at Target.com. The Apple store at Southdale offered a trade-in value of $168 ($166 at Apple.com). Apple just added trade-ins on iPhones. Best Buy's bid was $170.

On Aug. 12, the highest price offered was Gazelle.com at $200. The lowest was at $149 at Target.com (store credit, not cash).

Phones retain value

The good news for owners of iPhones is their unparalleled ability to retain value, said Karl Hattman, regional manager at Pawn America.

"An iPhone 4 without 4G is worth more than many newer Android phones," he said. In general, Androids depreciate about 10 percent per month, compared with 5 percent depreciation for iPhones, said Raman.

Consumers who don't want to do a trade-in through the mail via Gazelle, uSell, Glyde or other resellers can get store credit at an Apple Store, Target or Best Buy. Those who want cash can try an ecoATM kiosk at Eden Prairie Center, Maplewood Mall, Northtown Mall, Southdale and Rosedale, New Uses store in Minnetonka or a local pawnshop.

For an even better deal, try unloading it on Craigslist, where prices ranged from $150 to $350 for the 4s. On eBay, prices for 4s that sold on Sunday ranged from $151 to $400.

On the other hand, holding onto the old model until it dies saves the most.

The Washington Post contributed to this report.