Per hour, per project, per page, per line or per word

For those of you who offer transcription services, I am wondering how you bill: per hour, per project, per page, per line or per word. Also how do you deterime the amount of time it takes to transcribe an audio file or tape. Is it based on number of speakers, length of recording, quality of recording, etc. I have seen a couple of formulas: (1) 30 minutes of audio in one hour or (2) 4 to 5 times the length of tape.