Actually you would be upgrading to XR Mill expert..as XP series is a past version...
As for cost..you should call OneCNC and see..I don't know what it is as you are upgrading both a Version and from Professional to Expert..
As for is it worth it..well upgrading from XP Pro to XR Pro is well worth the cost..as there are a tons of new features in the XR series compared to the XP series. Then to upgrade from Pro to Expert would bring more tools and features.
The main differences between Pro and Expert (in both versions) are the Mold tools..Automatic Hole Blocking, Custom Tool pathing, Mold Splitting Tools and such..plus several more tool path options, Spiral, Plunge Rough, Radial, Steep Wall, Plateau Machining, along with the before mentioned Custom Tool.