By horrendously marking up their beer prices. From the story:
“According to local beer industry sources, the Mets’ and Yanks’ concessionaires purchase half-kegs in bulk for approximately $29 per half-keg, roughly half the cost charged to bars and restaurants. Each half-keg holds 1,984 ounces, or 124 16-ounce servings.
“So, if the teams sell 16-ounce servings for about $6.50, which they do, the profit over the initial purchase price of each half-keg begins when the fifth beer is sold and that profit, per half-keg, is nearly $800.”
Thanks to Al Tompkins for the link.