Coupon Code that Lowers Monthly Membership Fee

Currently the Pro Site Coupon Code automatically discounts the first month's membership. Would it be possible to make this so that the coupon code discount's the membership fee for the lifetime of the customer? For example we want our normal monthly fee to be $49. With the correct coupon code, however, we want users to pay $39 a month for their membership. I am pretty sure this functionality is not built in to Pro-Sites at this point but where should I start if I was going to code it for myself?