The four alternatives, plainly

Film book. A film book is the whole novel produced as cinema: fully cast voices, sound design, licensed score and motion imagery, cut scene by scene from the author's own text. Nothing is abridged and nothing is invented for it. It is the closest alternative to audio because it is built on the same cast performance — priced at 12% of word count against 7.000000000000001% for audio.

Novel-game. A novel-game is a playable edition of a novel: one gate per chapter, walked in the author's own world, built from a blueprint surveyed out of the manuscript itself. It reaches a reader who will not sit still for nine hours of anything, and it is the only one of the four a reader can lose.

Podcast serial. The same recording as an audio book, cut into episodes and released on a feed. Cheaper to distribute, slower to earn, and it competes with free.

Book trailer. Thirty to ninety seconds. It is a poster that moves. Useful, and not an edition of your book — do not let anyone sell it to you as one.

Which one for which problem

If the problem is 'my readers commute' — audio, and nothing else.

If the problem is 'nobody discovers me because I have nothing to show' — a film book, because it yields both the full edition and every cut of promotional footage you will ever need from one production.

If the problem is 'my series is finished and the audience has moved on' — a novel-game, because it is a reason to return to a world rather than a repackaging of it.

Asked and answered

Which alternative is cheapest?

An audio book remains the cheapest complete edition at 7.000000000000001% of word count. A film book is 12%. A novel-game is priced on a banded rate with a floor because it is engineered as well as produced.

Can one manuscript become all three?

Yes, and it should be produced in that order — audio, film, game — because each stage reuses the cast performance and the scene work of the one before it.