Tradeweb Markets Inc. , an operator of electronic marketplaces, priced its initial public offering at $27 a share late Wednesday, above its expected range of $24 to $26. The company will offer 40 million shares of its Class A stock -- a significant boost over the 27.3 million shares that were originally to be offered -- to raise just over $1 billion, with a valuation of about $6 billion. The company is expected to start trading on the Nasdaq on Thursday, under the ticker symbol "TW."