Probably by taxi cab ... but I will wait for others to respond since they may be able to tell you cost. There is also a Universal section here on the DIS boards and they may have some ideas.
We are going to the Universal Parks in Oct 2007 (still staying at WDW Resort) but we will have a car. I call it going to the "Dark Side" but Universal and Islands of Adventure do sound like fun!
Are you actually going to stay at one of the Universal Resorts?