Coinbase is finalizing a deal that would value the company at about $8 billion, a transaction that would make Coinbase one of the highest-valued startups in the U.S. The company is in talks with Tiger Global and its current shareholders for an investment of up to $500 million, according to people familiar with the matter. Coinbase is expected to add about $250 million in new money to its own coffers, the people said, and as much as $250 million more could go to buying out existing investors, though the exact amount has yet to been determined.