Not sure what type of ticket/class the OP is holding. But in general premium fares with CX is generally cheaper ex-YYZ than ex-JFK and especially for FC tix where fares ex-YYZ are more than 50% cheaper than those ex-JFK. Premium award availability are generally also easier to find on the YYZ flights than the JFK ones. So even adding the LGA-YYZ segment is not only still cheaper compared to the direct JFK flight, but often worthy for an award tix that might not be available on the JFK route.