Clearly you have comprehension problems, I said professional chefs mix Pecorino and Parmigiano, and indeed Grana Padano. In fact Luciano Monosilio—widely dubbed the "King of Carbonara"—uses a specific blend of Pecorino Romano and Grana Padano in his signature carbonara recipe. He incorporates Grana Padano to soften and balance the sharper, saltier flavor of the sheep's milk Pecorino. Her certainly has no need to "keep costs down", he mixes in Grana Padano for reasons of flavour. Grana Padano is slightly milder but the difference is hardly noticeable. They are both cow's cheeses, obviously the odd one out is the Pecorino which has a sharp and salty taste by comparison.