So choosing the right car for your teenager is probably something you'll want to sit down and think seriously about. Not only do you have to think about the actual physical cost of buying a car, but you'll need to think about the cost of car insurance - especially learner driver insurance if they're still learning.
The ideal car for many teenage boys tends to be a car with a large engine and an equally big spoiler. However, it's unlikely that they'll be able to find a learner driver insurance provider willing to cover them.
Besides - do you really want your 17 year old driving around in a powerful car? A teenager may appear confident but it takes a few years to gain the experience required to handle a car.
When considering which car to buy a good rule of thumb is a car with a small engine (i.e. 1.0 - 1.2) such as a Ford KA or Vauxhall Corsa. Its also worth discouraging your teen from modifying their car as modifications can have an impact on the cost of provisional car insurance.
However, it's not just about the cost of car insurance - you need to bear-in-mind other costs such as fuel prices, repairs, servicing, etc. With a smaller, modest car these costs tend to be lower.