Grocery stores figured this out years ago. It's by giving that secondary number a subordinate position on the price tag. Not by redesigning the price tag to list the actual options in a misleading way (and then making the customer do even more arithmetic—not less—in order to answer the question, "How much is this going to cost me TODAY?")

No one who is doing the "$X per month, billed annually" is doing it because they're trying to make it easiest for the customer. They're doing it because lower numbers on the price tag makes for more sales.