The chain and cassette wear down together, so I think most shops prefer to change both at the same time, to ensure smooth working. If you replace the chain early you can get away with not replacing the cassette until next time the chain needs replacing, however. They're actually really easy to replace yourself if you have basic tools. Parts will vary in price, usually according to weight.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett