Price varies wildly depending on the shape.
A standard size rectangular window, even a fairly large one, is like a few hundred bucks. The labor cost to remove and install is probably close to the material cost.
But once you get fancy w/ circular, rounded, those giant "gallery" view windows that pop out from the siding, etc... shit gets crazy expensive. Like well into the thousands per window.
Even if it's "only" $300 parts and labor b/c it's a basic rectangular window, it still would suck to replace, though.