I think you need a minimum salary of Qr. 7000/- if I am not mistaken to get visa for your family. I have no idea about your salary package. Are you getting family accommodation from the School over your monthly salary? Accommodation is the most expensive thing in Qatar. You should also consider other expenses like food, school, medicine, transportation, water and electricity plus communication. You should also consider the air fare which is quite big if you are going every year to your country. You should give us an idea what will be the package you are getting to advise you.