(Reuters) - Shares of SoftBank Group Corp-backed Compass Inc jumped 18% in their stock market debut on Thursday after being priced at the lower end of the reduced initial public offering (IPO) price range. Stock opened at $21.25, above the IPO price of $18 per share, giving the real estate brokerage firm a market value of $8.22 billion, and a fully diluted valuation of over $10 billion, which includes options and restricted stock. The company, which downsized its IPO and reduced its price target range, sold 25 million shares on Wednesday, raising about $450 million. It was earlier looking to raise about $936 million by selling 36 million shares at the upper end of its initial price range of $23 to $26 per share.