Thanks, Jerome. I've now created a 'characteristic' called 'Subscription period", added it to the product and then created the variant prices for the three subscription period options, monthly, annual and biannual. So now customers can choose between lengths of subscriptions, which is great. I still need to know how to style the way the price is displayed.
Common price: US$1,440.00 Our price: US$120.00 each
That formatting is appropriate where a merchant sells an item for less than the recommend retail price; however in the context of annual subscriptions it is weird, if not misleading.