You'll have some people coming here saying they have a third-party adapter and it works fine for way cheaper. Good for them but you also risk any damage that could be done to your computer (those that have third-party adapters will disagree). You'll get mixed reactions so it's up to you whether or not paying the Apple price is worth it.
I recommend buying from a company called MacBookAdapter.com. They refurbish genuine Apple chargers, to make them cheap. You can get a 60w charger for $30. I bought 2. One of them died about 2 months after purchase, and they sent me a new one with a prepaid mailing envelope to send back the defective one. They are a very good company and recommend them to everyone.