Bermuda-based White Mountains Insurance Group Ltd will pay $408.5 million to buy back more than 800,000 shares in a tender offer.The $500 per share offer expired at midnight last Thursday. The company made the offer to buy back up to one million of its shares at the $500 price after buying out billionaire investor Warren Buffett’s $45 million stake, a move announced on February 22.Based on a preliminary count by the depositary for the tender offer, 817,022 shares were tendered, the company said.White Mountains paid a total of $408.511 million, excluding fees and expenses, for the shares.The final number of shares purchased will be announced following the expiration of the guaranteed delivery period and completion by the depositary of the confirmation process.The company expects to have approximately 6.63 million common shares outstanding after tender offer purchases.