AT&T detailed pricing for its first Windows Phone 8 devices, with the US operator giant set to price Nokia’s Lumia handsets aggressively.

When purchased with contracts, Nokia’s Lumia 820 will be available from US$49.99, with the Lumia 920 from US$99.99 – and a free wireless charging plate will also be bundled with the more expensive device for a limited period.

In contrast, HTC’s Windows Phone 8X will be available from US$199.99.

Away from Windows Phone devices, AT&T’s portfolio includes the iPhone 5, which is available from US$199.99 with a two-year contract, and US$199.99 for the Galaxy S III.

The pricing, along with a slight headstart over HTC (the Lumias are available from later this week, while the HTC is set for launch “before thanksgiving”), will be important for Nokia to build its market share in the US, where it has recently seen its numbers on the slide.

In Q3 2012, it shipped just 300,000 units in the North America region (including Canada), half of the 600,000 it moved in Q2, due to reduced demand for earlier Lumia devices powered by the legacy Windows Phone 7.5 platform.

The company has also recently announced a device for Verizon Wireless, marking its first real play in the US CDMA smartphone market, meaning it will have devices available from three of the “big four” operators – T-Mobile also has WP8 Lumia devices in its portfolio, while Sprint is the odd-man-out.

In the same period, the company shipped fewer than 3 million Lumia devices worldwide, compared with 4 million in the prior sequential quarter.

As Nokia transitions its portfolio from the legacy Symbian and aborted Meego OS to Windows Phone, the company has seen its once leading smartphone market share eroded by a raft of rivals.

According to IDC, it did not even rank among the top-five smartphone vendors in Q3, having been passed by Samsung, Apple, RIM, ZTE and HTC.