SAN FRANCISCO — The fees to attend California’s community colleges are by far the lowest in the United States — less than half the national average — yet at least 40 percent of the 2.4 million students in the largest U.S. system of higher education do not pay them.

The reasons? An unusual financial aid program and a half-century-old vision that made affordability and open access the chief purpose of the two-year schools.

The costs are so low at $1,380 a year for a full-time load and are so frequently waived for students who meet income and social eligibility criteria that during the recent economic downturn the system relinquished more fee revenue — $577 million — than the $361 million it collected, according to 2011-12 data from the California Community College chancellor’s office.

After steep state spending cuts forced colleges to reduce course offerings and turn students away, college leaders, lawmakers and experts are questioning whether California can afford such generosity.

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