A. You will need to see a dentist for costing as each case has many variables.  As far as what causes gum disease there are 3 factors- your immunity, the bacteria in your mouth and how well you clean your teeth.  If you do not clean all the surfaces of your teeth including between your teeth and your predisposed to gum disease because of your immunity then the bacteria causes a body response that removes the bone supporting your teeth – hence your loose tooth.  The first thing you need to do is to see a dentist to have a full diagnosis and see there hygienist to begin your gum treatment.  Good luck.