The free versus paid question in test prep apps has a simpler answer than the app stores want you to believe: start free, and pay only for what free can't do.
Free genuinely covers a lot. Duolingo's free tier handles daily English exposure and streaks. Anki costs nothing on desktop and Android and remains the best vocabulary tool at any price. Quizlet's free tier gives you shared flashcard sets. The BBC Learning English app is free and excellent for listening. An IELTS TOEFL daily task app free tier can absolutely carry your vocabulary and daily consistency needs without you spending a cent. I've seen students reach 6.5 on free tools alone. It's possible.
What free usually can't do well comes down to three things: graded writing feedback, realistic full length mock tests with scoring, and personalized study sequencing. Those cost money to produce, so they live behind paywalls. The question is whether you need them from an app specifically. Graded writing feedback is the strongest case for paying, because grading your own essays is nearly impossible. You cannot see your own blind spots. I say this as someone who's tried. Everything else, mocks included, has decent free alternatives if you hunt.
The trap to avoid is paying for motivation. A paid app with streaks and badges is not a better plan. It's a more expensive version of the same dopamine loop. Before you subscribe, ask what the paid tier does that changes your study decisions. If the answer is just "more questions," keep your money. Question banks don't raise scores. Targeted practice on your weakest section raises scores.
So my take: free apps for daily reps and streak tracking, and spend money (if you spend any) on human feedback for writing and speaking. The sequencing of what to study when doesn't need to come from an app at all. The 8-week study plan at internationalstudyhelp.fyi handles that part, so your apps only have to handle the reps.