"... the thought of money coming out of my account each month until I cancel (which I would probably be too lazy to do)" I think you've answered your own quest

"... the thought of money coming out of my account each month until I cancel (which I would probably be too lazy to do)" I think you've answered your own question here. Recurring monthly subscription models work so well precisely because we're passive by nature. You'll earn more revenue by going monthly. The other attraction of the monthly fee is the headline figure is less off putting to the end user - "Hmm, $9 that doesn't seem so bad" compared to the equivalent yearly charge ("$108 bucks, no way man.")