Yahoo (NSDQ: YHOO) investor Ivory Investment Management went public today with a plea to the board to sell the search business to Microsoft (NSDQ: MSFT) for what it estimates could be $15 billion upfront. MarketWatch reports that Ivory, which owns a 1.5 percent stake in Yahoo, issued an open letter to the board urging it to salvage the deal with Microsoft “and not miss another value maximization opportunity.” Ivory’s proposal sounds familiar: sell search to Microsoft and then make Microsoft Yahoo’s search partner. From the letter: “We believe a search deal with Microsoft could deliver value to Yahoo shareholders of $24-29 per share, or more than double yesterday’s closing price of $12.19.”

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Update: MarketWatch has updated the story with more detail. Ivory suggests that combining the two would save $800 million in duplicate costs and that a single unit could increase revenues by at least 20 percent or $500 million a year. On its reasoning: “On an after-tax basis, the $15 billion payment from Microsoft would be $9 billion for Yahoo shareholders, leaving Yahoo with $21.2 billion of cash and investments (up from $12.2 billion today) and annual EBITDA of $2.4 billion (up from the midpoint of current guidance of $1.9 billion). Applying a 5x EBITDA multiple on the “new Yahoo” would result in a value of $24 per share. If Yahoo were to go a step further and deploy the $9 billion in new cash for its own shares at a $16 offer price, it could reduce its share count by 40% which would leave the remaining shareholders with a stock approaching $30 per share (amazingly close to the original bona fide bid from Microsoft).”