WASHINGTON – It seems like every few weeks or so, a new cellphone model is introduced.

And those thinking of upgrading should do it soon. Waiting just 30 days after a new model is introduced could significantly depreciate the phone’s value as a trade-in, according to the Star Tribune.

Apple recently unveiled the 5S and 5C, and analysts told the Star Tribune that old iPhones will lose 5 percent of their value by the end of the first weekend, 12 percent within two weeks and 20 percent within a month.

Nik Raman, with uSell.com, which helps people sell their old cellphones, tells the Star Tribune that consumers can save $70 by timing the sale of used iPhones to just before or after the new models are unveiled.

For the best deal, the Star Tribune says Craigslist had the 4S selling from between $150 and $350. But on Ebay, they sold for as much as $400.

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WTOP’s Alan Etter contributed to this report.