The Lumia 520 certainly follows Nokia's previous strategies of having comparable devices at each price point, and invariably the halo effect of marketing around the high end devices helps the sale of the lower end devices. With one of Windows Phone's key strengths being the usability of the OS on lower specification devices, this has helped create the conditions for the 520 to succeed. And of course it helps that the 520 is a really nice device.
The more Nokia push Windows Phone down into this price territory, the better it will do, I suspect - budget Android phones tend to be slow and clunky, whereas the 520, on the whole, flies. And with greater sales at the budget end will come marketshare increase and revenue, increasing awareness further up the price spectrum. Assuming that Windows Phone continues to grow, I suspect we'll be looking back in a year's time and realising just how much the Lumia 520 and 620 had to do with the ecosystem taking off across the world.
Of course if you want to look at it another way, even with these comparatively huge volumes of Windows Phone devices going out of the door, it only shifts the smartphone market share needle a few percentage points. While a lot has been done by Microsoft, Nokia, and the carriers with the Lumia 520, there is a lot more to be done for the rest of 2013 and into the next year.
So a nice pat on the back, but there's another mountain round the corner.