Hello sirius and welcome to the forum.
...Somebody told me that you can buy the Captivate from amazon and it will cost around $200 including a contract...
Yes you can for ~$200 with a 2year contract.
...that i can bring it to india and unlock it and then use it in India with my existing gsm sim card..Is this possible?
There is an app called SGS Unlocker which you can get from the Android Market and it will help you unlock the phone and let you use another companies SIM card. HOWEVER, I don't know what bands the company in India uses OR of they will be compatible with the Captivate. Finding out that information will inform you about whether it will work in India and how much it will cost you.