Well according to Get Away Today (reputable source) an 8 day ticket costs $244, so whatever you paid for your 5 day ticket take the difference and that's how much you have to pay. However, you may want to look into just upgrading to an AP (annual pass) which is only $259 (for the Deluxe) and gets you discounts (10-15%) at pretty much all restaurants at the resort. Those discounts can add up a lot if you plan on going to the parks 8 days. It may be worth it. But do not upgrade to an AP until after you use your Magic Morning!