Nokia and T-Mobile rolled out the Lumia 710 and clearly plans to use price---$49.99 after a $50 rebate---as a weapon to grow market share. The goal: Target the feature phone crowd that wants to trade up to a smartphone.
Also see: Great Debate: Can Windows Phone 7 and Nokia become players in the U.S.?
As noted previously, Nokia and T-Mobile aren't exactly the strongest tandem when it comes to invading the U.S.
However, price may change that equation. The Lumia 710 is a value at that low price and Microsoft gets more exposure for Windows Phone. Microsoft and Nokia still have challenges since Android and Apple both have cheap phones too, but the Lumia 710 price might be right to gain market share.
In the end, Microsoft can't avoid the Windows Phone ecosystem question based on price. The iPhone 3GS is free and has a much stronger app selection.