~100,000 deaths X ~70% Social Security and Medicare Beneficiaries X $40,000 per year = $2.8 Trillion
Obviously any of these three estimates could be way off, but this gives us an order of magnitude look at the potential numbers.
PS. Doctors have told me that there were studies which calculated that smoking was keeping us solvent because it meant far less entitlement payments (especially for end of life care) to the elderly.