That's an interesting angle. And then poorer people will host taxi services for richer people. Oops, cars are fungible, which means charging based on the user's income will not work.
Right, the best you could do would be to base it off of the car model associated with the license plate. But then you might just get people driving their old beaters into the city, which would have worse emissions than their newer cars and be worse for everyone who is near them on the street.