By Svea Herbst-Bayliss BOSTON (Reuters) - Activist investor Engine No. 1 spent roughly $12.5 million to win three board seats at Exxon Mobil Corp (NYSE: XOM ), less than half its original budget, for the year's biggest and most closely watched corporate contest, people familiar with the number said. Engine No. 1 in May shocked the oil-and-gas industry when Exxon shareholders, frustrated by weak returns and the U.S. leader's flagging attention to climate concerns, elected three of its four nominated directors to Exxon's board. Investors said the fund's small budget could become a template for low-cost proxy contests. The victory against one of America's most iconic companies is even more impressive considering Exxon's market valuation of $265 billion, industry analysts said. Exxon had said it planned to spend $35 million more than its usual proxy solicitation costs, but did not specify a figure.