The cassette sprockets are rusty for sure, however, there does not seem to be too much wear. I don't see sharks fins or hooking on the tops of the sprockets. This may be salvageable. Is the hub still good? If so, there are several methods to remove the rust:

Evaporust
Wire Wheel
Sand Paper
Vinegar

What I do is to grind off the rivets on the largest sprocket to break the cassette down to the individual sprockets. The you can clean them up. The rivets are not needed to reassemble. They were only there to make assembly easier in the factory.