You should have thrown this question before accepting the offer. Approximate breakdown of monthly expenses;
2 bedroom apartment QR 7000
Grocery QR 2500
Schooling for one child QR 1500
Car Installment and fuel QR 2000 (Saloon car only)
Dinner once a week QR 500
Internet, TV and telephone (landline) QR 250

It usually takes a month to arrange family visa after you visa is stamped.