How much to charge for English to Japanese song translation?

Hi, everyone. I need some advice. Someone wants to commission me to translate their original Vocaloid song into Japanese, but I don't know how much to charge.

The person is a popular English Vocaloid producer. I worked with them about a year ago, but it was before I was into the vocal synth hobby and did it for free, because the song was about a minute long and I had never done anything like it before. This time, they are willing to pay me for a longer song.

I don't know if I should charge per hour, per unique song line, a flat rate per song, etc. It takes me about 3 days to finish a song, because I have to edit the translation to fit with the available syllables and give it a matching feeling. I've been practicing translating random people's songs but not uploading them, and I tend to think about the song all day even when I'm away from the computer cause I love translating so much, I feel like a per hour rate would be unreasonable due to this. >>;

They would send me the English VSQ, I would overwrite it to be in Japanese. They would get kanji and romaji lyrics from me. No tuning from me, just lyrics. So, how much is reasonable/how much would you pay?

Thank you so much for your reply. I ended up putting a lot of thought into it, comparing rates of Japanese translators, doing some math, and I finally came up with what I think my time is worth as you said. (I decided to charge per unique word, since songs repeat a lot.)