Cost of transportation to Universal from Disneyworld

Can anyone tell me a little more about the various ways to get from Disneyworld (POR) to Universal Studios? We will not have a car available to drive ourselves. We are only doing the one day at Universal and most of our time will be spent doing the Harry Potter attractions and also Jurassic Park area. How much do the various options cost and do they need to be scheduled ahead of time? I know that we could take a cab over and back and could just call when we are ready to head back to our resort but am wondering about the cost. I also know there is some sort of ticket package available that has ground travel included but from my understanding you have to reserve a specific pickup times. My concern with this would be that we might be ready to head back to our resort and then have to sit around waiting until our pick up time....or we might want to stay later than our scheduled pickup time and not be able to. Anyway just trying to look at all possible travel options available and will appreciate any thoughts or opinions on ways to travel over there and back.