with the limited information you have provided in your question, its not possible for one to give you a firm answer.

You havent mentioned if you 6500 is your basic or complete package.will you be provided with accomodation, car, medical etc.

what field are you coming into & your years of experience to judge if the offer is worth & you are being treated fairly.

I would suggest you to Search this Site (on the upper right) & you will find plenty of replies & comments on Cost of Living in Qatar that will help you.

Best of Luck!