Shares of the company, which has won a cult following for its rich milkshakes and hormone- and antibiotic-free burgers, priced at $21 each, an underwriter said, valuing the chain that started as a hotdog cart in a park at about $745 million. The IPO market has been particularly fruitful recently for so-called fast-casual restaurant operators hoping to replicate the scorching growth of Chipotle Mexican Grill Inc (CMG.N). Shares of Habit Restaurants Inc (HABT.O), another gourmet burger chain, have risen more than 80 percent since their market debut two months ago.