You're assuming that people are willing to pay for your product up front for 12 months. If your price point is so small that an annual subscription is under $20, then it doesn't really make sense to charge monthly. I was speaking mainly for higher priced items.
Both are good points, would definitely like to see more research on the topic. I definitely agree there is a threshold when it comes to monthly vs yearly. I know you just tossed out a number but $20/year seems a little lower than I was thinking.
