I may be missing some details concerning what you want to do as the pricing does not sound quite right however, assuming you have a US PayPal Business Account, you can use the online button creator to generate the code you need - you can create a drop down selection for a monthly and annual option. Note, this would only be for one subscription item with two choices.
If you're intent was for two different subscription items managed by "one" button, that would be a different scenario and not doable with Payments Standard.
