screenshot by Lance Whitney/CNET

You can score as much as $300 if you buy a new Moto X or Moto G smartphone and trade in an existing phone with the right value, courtesy of a new deal from Motorola.

Available through the company's trade-in page, the promotion works as follows:

Enter the brand of the phone you wish to trade in. The page will show you the amount of money that you can score by trading it in. Buy a new Moto X or Moto G phone. Send in your old phone within 30 days, and Motorola will mail you a Visa prepaid card, which you can use as a regular debit card on any type of purchase.

Motorola unveiled the latest editions of its Moto X and Moto G phones at a launch event on Thursday as its latest entries designed to compete in a mobile world dominated by Apple and Samsung. Both devices are designed to undercut the competition on price yet still offer features typically found in high-end and pricier phones. The Moto X will officially reach consumers later this fall, while the Moto G is available as of Thursday.

So how much could you ultimately get back on the cost of a Moto X or Moto G phone?

Well, the unlocked version of the Moto X will retail for $500, but a carrier-subsidized edition will sell for just $100. The Moto G is going for $180 unlocked. And the Visa card can carry an amount as high as $300. So does that mean you could potentially get back more than you paid for the phone? Yes, at least according to Motorola.

A Motorola spokesman told CNET that a situation in which a buyer would get back more than the amount paid for the Motorola phone would be unusual as it would require a device with a very high value to be traded in. But it is possible. The spokesman also noted that subsidized phones are underwritten by the mobile carriers.

Here are a few examples: When I ran through the trade-in page, phones from BlackBerry, Nokia and LG typically resulted in low quotes - under $100. But more-popular phones netted higher amounts.

A fully functioning, undamaged 64GB iPhone 5S scored a $280 debit card. A fully-functioning, undamaged Samsung Galaxy S5 maxed out at $300. Of course, a device in poor condition would earn far less. For example, a Galaxy S5 with a damaged screen would be worth only $60. One with liquid damage would be worth nothing.

Still, the overall deal shows the lengths to which Motorola is willing to go in order to get the new Moto X and Moto G into the hands of the public.