We're not here to judge whether this number is good or bad, but Elbert Perez, who has no less than 12 games on Windows Phone (e.g. Quadra, Impossible Shoota, Nom Nom Worm, Steam Castle, etc.), just posted how much he's brought in so far. See his site here.
Now, a few things to remember abou Perez: he's basically one guy (indie) doing all the developing, which is quite impressive and his games are all free with ad support. It was five months ago he switched from paid games at about $0.99 to making them all free with ads. So is $30k in ad money pretty good? We think it's not bad but it obviously shows that the Windows Phone platform needs to grow in order for him to continue to make and increase revenue.
In many ways for developer's, Perez's model of game development and distribution should be a source of inspiration. We recommend checking out his "Going Full Time Indie" piece on what it's like to not work for a big studio.