Well, this is going to be pretty vague but when I UuA from LHR - PEK recently it was about £40, and when I did the same on LHR-JNB it was about £35-ish from memory. Both of those are considerably longer flights than LHR-JFK (although I appreciate that these things aren't entirely linear).

I don't understand how it's calculated - some elements (is it the YQ?) is the same for both classes but others (taxes?) do increase. But I've always been very happy to pay a relatively small amount for such a huge benefit.