Yeah, you should be doing more than 20.000 kms a year before a diesel gets cheaper than petrol. But I do drive a lot and next year I'll be going to school, that would be about 50 kms a day. So I'll probably be driving a lot.
And honestly, I don't care about the emissions or whatever, I'll have a pretty high economy car, so why would I care? It's the people with the big heavy cars that should start thinking, or eg. more than half the population of America.