From what I see the code downsamples video to 5 fps, so 1 hour of video is 3600 seconds * 5 fps = 18,000 frames. 18,000 frames * $0.00079/frame = $14.22. A couple dollars more with the overlap.
(The code also tries to skip "still" frames, but if your video is dynamic you're looking at the cost above.)
(The code also tries to skip "still" frames, but if your video is dynamic you're looking at the cost above.)