We teach Japanese for a living, so read this with that in mind. It is the honest version anyway, because a student who enrols for the wrong reason stops coming.
The short answer
Information is free. Feedback and consistency are not.
Almost everything you need to pass JFT-Basic or the JLPT — grammar explanations, vocabulary, kana, listening material, sample questions — is available at no cost and in abundance. What money buys is somebody noticing when you are wrong, and somebody noticing when you stop.
What is genuinely free and genuinely good
Official sample questions. The Japan Foundation publishes JFT-Basic samples on its own site. These are the only definitively representative material in existence. Start here.
Spaced repetition software. Anki is free and is the best tool of its kind.
Graded listening. A large amount of A2-level Japanese audio is freely available.
Kana resources. Learning both scripts costs a fortnight of attention and nothing else.
Our free practice test. All four JFT-Basic sections, scored by section, with an explanation for every answer in six languages. No sign-up, works offline on a cheap phone.
That set will get a disciplined learner with a foundation to 200 points. See free JFT-Basic resources.
What is worth paying for
Correction. Free material tells you the answer. It cannot see that you have been misreading a particle for a month, or that you are marking yourself right on listening items you actually guessed. That blind spot is the most expensive thing in self-study.
Structure, if you cannot build it. A syllabus that does not let you over-study what you enjoy. Most self-studiers over-invest in kanji and vocabulary — which feel productive — and under-invest in listening, which does not.
Accountability. The real product. Almost every failed self-study attempt ends in week five, when the novelty is gone and nobody notices whether you studied. See staying consistent.
Official past papers, where they exist for your exam and level.
What is not worth paying for
Information you can get free. Grammar explanations, word lists, kana charts. Paying for these is paying for packaging.
Question banks without explanations. A bank of thousands of items that only marks right or wrong trains you to recognise those items. The explanation is where the learning happens.
Anything claiming an official word or kanji list. There is no official JFT-Basic list, and the JLPT has published none since 2010. A product misrepresenting this is telling you something about the rest of it.
Guaranteed passes. Nobody can offer one.
A second grammar book. The first one, unfinished, is not a resource problem.
How to decide
One question:
Am I missing information, or am I missing follow-through?
If it is information, you almost certainly do not need to spend anything. Read can you pass by self-study and get on with it.
If it is follow-through — you have tried alone and stopped, more than once — then that is the thing to buy, and it is a legitimate purchase. Our classes are capped at eight students specifically so that an absence is noticed, which is the actual mechanism.
The cheapest thing you can do
Do not book a test before you are ready. A retake is the full fee again, plus the travel again, plus two more months — and the failure was predictable for free.
Two full-length timed tests in the same week, both at 200 or above. Then book. See what JFT-Basic costs.