One of the first new programs in the Obama administration’s massive health care overhaul will start next month in California — an insurance plan for people with pre-existing conditions who have been denied individual coverage. The new $761 million program funded by the federal government will be overseen by a state agency in Sacramento. On Thursday, the agency set the cost of monthly premiums for the Pre-Existing Conditions Insurance Plan. Premiums vary by age and by county, and are guaranteed not to exceed standard premium rates for those populations. San Diego premiums are among the lowest, ranging from $127 a month for people under age 15 to $904 a month for those over 74. Applications can be filed this month for coverage to begin in September.