Assuming it still lasts long enough for you, you can keep using the old battery until it doesn't.
The guideline from Apple about the battery lasting at least 1,000 cycles is a rough estimate. From my experience, batteries can last as little as 500 cycles and as many as 1,500.
If you're willing to repair it yourself, [product|IF123-099|the battery from iFixit] is only $110 for the part, or $120 if you don't have tools and need some. [guide|125636|Here's the guide for replacing the battery.]