The price of SCJP exam varies in different parts of the world. Contact your nearest prometric center to know the cost. You don't need any voucher to give the exam now, you can directly pay your prometric center...
@ Ankit: thanks. I found the cost of SCJP. Its $180 (after 40% discount from SAI. The actual cost was $300).
1. You need to enroll to SAI program. webpage. Yo just need to send your full name, your university/Institution name (Your university/Institution should be a member of SAI institute) and country name to Contact_SAI@sun.com.
2. Once you are registered with the SAI, send the same information to Contact_SAI@sun.com. They will also need some documents which verifies that you are a current member of University/Institute. (Like photo ID).