Toyota ( TM) is assessing the cost of closing a California auto plant it previously shared with General Motors and is leaning toward shutting the plant instead of operating it on its own, a report says. Toyota is likely to close New United Motor Manufacturing, or NUMMI, since the cost of keeping the factory open will rise, Bloomberg reports, citing people familiar with the plan. TheStreet.com reported at the end of June that GM was pulling out of its 25-year old joint venture with Toyota in which the two companies shared the Fremont, Calif., manufacturing plant. Bloomberg reports severance packages for the factory's unionized workers would be a substantial portion of the overall closing cost for Toyota. A spokesman for Toyota's U.S. sales unit told the news agency the company hasn't made a decision on the fate of the plant. But a decision on keeping or closing the plant could be made as early as next week, Bloomberg reports, citing one of the people.