Hi,
Adding to Shweta's comments, performance-linked pay is the result of your Performance Appraisal process. The objectives set are measured against the results achieved. In an organization where the individual KRAs are linked to the business objectives, this will very clearly entail understanding the individual contribution of the employee to the organization's plans and success.
As pointed out by all, this is not fixed in nature. In India, the senior management team's variable pay is accounted for around 20-25% of their CTC, as per my knowledge. This percentage goes down with levels going down.
As very rightly pointed out by Shweta, the quality of performance-based pay is directly linked to the PMS adopted by the organization. The transparency and objectivity of measuring the performance will be deciding factors for a successful performance-related pay.