The government could save $274.1 billion in medical costs in the year 2000 by regulating fees and replacing private insurance companies with universal public coverage, consumer groups said. A report by Families USA Foundation and Citizen Action said doctors and hospitals could cuts costs by eliminating unnecessary treatment and holding down increases in providers' income. A spokesman for the Health Insurance Assn. of America, a private insurers' group, said in a statement that a system funded and administered by the government would lead to health care rationing and would be costly.