Individuals buying health insurance on their own will see their premiums tumble next year in New York State as changes under the federal health care law take effect, Gov. Andrew M. Cuomo announced on Wednesday. State insurance regulators say they have approved rates for 2014 that are at least 50 percent lower on average than those currently available in New York. Beginning in October, individuals in New York City who now pay $1,000 a month or more for coverage will be able to shop for health insurance for as little as $308 monthly. With federal subsidies, the cost will be even lower.