The $12.5 beef buffet you're referring to is likely at "Siga La Vaca," a popular all-you-can-eat parrilla (Argentine grill) restaurant located in the Puerto Madero district of Buenos Aires. This restaurant is known for its extensive buffet that includes a wide variety of meats, salads, and sides, offering a fantastic value for the price (Tripadvisor) (Nomadic Foodist) (Trip).
If you're in Buenos Aires and looking to enjoy a hearty and affordable beef buffet, Siga La Vaca would be a great choice. Make sure to check their current pricing and opening hours, as these can sometimes vary.