Amazon intends to keep its position atop the e-reader mountain, announcing Wednesday that it has struck a deal with AT&T to sell its ad-supported Kindle 3G for just $139. If you don't mind a few ads popping up on your Kindle, you can now get the convenience of free 3G mobile networking for the price of a WiFi-only Kindle.

Amazon has been finding ways to drop the price of its popular Kindle devices since its introduction in 2007. Last year, Amazon slashed the price of its 3G-equipped Kindle 2 from $259 to $189. Last month, it began offering a third-generation Kindle 3G with Special Offers, which saves users $25 in exchange for being shown ads on the device's screen saver and along the bottom of the home screen. The announcement today shaves another $25 off the original $189 price thanks to AT&T's sponsorship.

"Kindle 3G is by far the fastest-growing connected device on the AT&T network," AT&T Mobility CEO Ralph de la Vega said in a statement. "We consistently hear from customers how much they enjoy the freedom of not being tied to a Wi-Fi hotspot and instead being able to download and read new books anytime, anywhere on AT&T's fast, reliable network."

Carriers have often subsidized devices like smartphones and netbooks to increase its subscriber base. What's unusual here is that the Kindle 3G comes with free 3G mobile service for the life of the device. AT&T is essentially kicking in $25 per device upfront to gain more users without increasing its average revenue per user (ARPU).

With its black-and-white-only screen and less-than-ideal Web browsing experience, Kindle devices typically consume far less data than the average smartphone or laptop. Whatever the cost, though, Amazon is footing that bill. While it was assumed that the higher price of the Kindle 3G accounted for whatever Amazon was paying for 3G service, it seems more likely that Amazon is paying AT&T some small amount per activated device on a monthly basis.

Neither Amazon nor AT&T responded to our request for comment on the matter. However, we believe Amazon is banking on eBook sales to cover these ongoing costs, while AT&T is banking on its $25 investment to generate a return over time. Regardless, Kindle 3G buyers can now get their hands on one for a pretty good price.