Let's do some basic arithmetic:
How many Veyrons are expected to be built? 150? Maybe 200?
Now, how many Toyota Priuses will be sold by the end of this year alone - around 1,100,000.
Ok, next, let's talk about the average mileage a Veyron is expected to cover in a year... I bet Simon Cowell's won't even do 10 000 km in a year but ok, let's go with that.
The average Toyota Prius mileage will, at very best, be twice that, if not 3 times as much. But, in the interests of fairness let's say it's 20,000 km.
A Veyron egests 571 g/km of CO2; a Prius a commendable 104 g/km...
So: Number of Cars x Ave Mileage x C02 output
If all the Veyrons in the world do as much as 10 000 km each, the combined CO2 emission will amount to 1,142 tons per year.
If all the Priuses in the world do as little as 20 000 km each, the combined CO2 emission will amount to 2,228,000 tons per year.
Bad supercar. Naughty supercar. Sit! Stayyy...