Surface sells for a much higher average price than the OEM version of Windows -- it retails for anywhere between $499 (for the lower-powered Surface RT) and $999 (for the highest-end Pro version), and that's not including the keyboard/cover.
Let's assume that Microsoft leaves a reasonable margin for its retail partners, including its own stores. Some analyst estimates put Surface Pro sales around 400,000 for the quarter, and Surface RT sales at maybe 200,000. Let's say that Microsoft collects an average of $700 per unit, including attached peripherals like the cover.
That would be $420 million. That's about 10% of Microsoft's adjusted Windows revenue of $4.62 billion (that's adjusted downward to make up for how Microsoft accounts for upgrade sales made in previous quarters). Even if those estimates are generous and the number was lower -- say 5% -- it means that Surface was material in helping Microsoft make up for a drop in PC sales this quarter. And those estimates could very well be low, which would mean that Surface was an even bigger factor in saving the quarter.
Charles Arthur breaks down the Microsoft Q3 numbers nicely and shows that Surface saved the quarter for Microsoft. But if the consumers are not coming back combined with slow sales of Surfaces, future quarters will look different.