Welcome to CodeRanch Tudor.
You are showing exam prep seminars in your first post. You do not need to take any exam prep seminars. However you do need to know the material.
I suggest that focus on Java 8 that is unless you have a particular reason to learn Java 6 or Java 7. I mention this because your first post shows Java 6, Java 7 and Java 8.
Many people purchase a book or two as a study guide and once they think that they can do okay they schedule/take the exam.
You can see how others have prepared and what materials they used here
https://coderanch.com/wiki/659980/Ocajp-Wall-Fame.
This could save you time, money and frustration.
There are also some free resources which you could find/use like the official Oracle Java Trail for instance.
The Java API and language/documentation could also be used, but I would not recommend that as a study resource as the information can be difficult to follow.
With regards to exam pricing, the prices are set by Oracle so you are left to pay what they want to you pay. However, the price for beta exams is less then the regular exam.
Beta exams are usually available for a short period of time only when the product version changes. You can only take the beta exam once per a product and these exams usually cover much more then the regular exam.
With Java 9 being a recent release you can probably expect the beta exam to be out some time in 2018. However there isn't too much in the way of learning materials/study guides for Java 9 published yet.
Aside from that Oracle has said that they are changing the release schedule for Java starting with Java 9 and many people do not know what that means for Java 9 exam/certification.