SANTA CLARA >> The tab to cover all that extra light rail and bus service to events at Levi’s Stadium is in, and it’s too hefty for the Valley Transportation Authority’s tastes.

VTA officials this week said it will cost them more than $3 million a year for 30 events, money they hadn’t banked on spending and an expense the transit agency says is “not sustainable.”

Officials have approached Santa Clara to ask for help covering the bill, but the mayor said the city can’t. So the agency is considering a whopping increase from $8 to $20 for a round-trip ticket for its express bus service that carried about 1,000 fans to San Francisco 49ers games and other sellouts such as the Sharks game and WrestleMania, although even that will not cover the extra cost.

Federal law prohibits the VTA from passing a similar fare hike to regular light rail or bus riders, so those would not be increased. An all-day fare costs $8.

“We’ve asked Santa Clara to look at their options but they’ve not committed to come back with a solution that we hope would help defray the costs,” VTA General Manager Nuria Fernandez said. “So we’re are looking at interim steps we could take.”

The VTA request caught Santa Clara by surprise. Mayor Jamie Matthews said former General Manager Michael Burns told him the extra riders would likely cover the bulk of added costs of getting them to Levi’s Stadium. An average 8,000 riders have taken transit to each event, and 16,000 rode to the outdoor hockey game between the Sharks and Los Angeles Kings.

“I was a little surprised when this issue came up,” Matthews said. “We’re always willing to listen, but we can’t contribute. If VTA cannot maintain its existing service, we may need to have the private sector step in.”

The $1.3 billion NFL stadium’s first year proved to be a learning period for local transit officials on ferrying fans to the games. Packed trains and long lines at ticket machines made trips home a long, slow journey. The VTA began more regular buses and added staff. It ran 80 light-rail cars on game days instead of the usual 20 that serve riders on the weekend. And 78 extra buses were used to carry extra passengers.

Plus the agency will spend $660,000 to build seven new ticket machines at light rail stations to reduce the cost of having staff workers selling tickets for stadium events.

By the end of the football season, VTA was operating six express bus routes providing 33 pre-game trips to the stadium using 25 buses from Fremont, Gilroy, Eastridge, the Almaden Valley and Cupertino. Charging $20 for a round-trip instead of $8 would bring in an additional $40,000 per event if ridership would not decrease due to the fare increase.

But that is still well shy of the $109,000 per event it costs VTA for each activity at the football stadium. And it costs the VTA $38 for an express bus ticket, so even with raising fares to $20, Fernandez said, “we’re still $18 short.”