This is an ill argument: the cost of buying a new cargo bike may be in the same ballpark of a badly used small car, but the cost of running it is negligible.
Try to sum up the following yearly costs:
gas
insurance
property tax
maintenance
parking
You end up with something like between 1k€ and 3k€ each year. Multiply this by the years you want to keep your (badly used) car, and then compare the total price