It's a fair price, and I know people that this approach has worked for.
Whoever, those examples were a few years ago, and in the interim, there seems to have been an increase in the involvement of HR departments in trainee driver recruitment. That means more box ticking, and less common sense, so I'm not sure a test pass would be as helpful as previously. The companies are increasingly concerned about discrimination claims, and are making sure that they cover their backsides accordingly. They need to be able to demonstrate that they have treated every applicant equally; you can be sure that they will be able to prove that they have.
If you can afford the money, it won't do you any har, but the advice at the bottom if the advert is very honest.The test pass absolutely doesn't guarantee you a job; there are many other factors that will be considered before you are employed.
Before you spend the money, have a read if the Structured Interview questions, which have been listed on here before. Can you honestly say that you have experience in those areas? If you haven't, you are wasting your money.