As with all ship communications, it can depend on the location of the ship. For instance we had terrible internet access on an Alaska cruise a couple years ago.
I was on the Oasis last December for it's first revenue cruise and access was fine. Nothing blazing fast but sitting in my cabin, or poolside, or in the Central Park with my Dell netbook I had wi-fi whenever I logged in. Probably 256k speed most of the time. The pricing plans are the same as using the internet cafe - which by the way is terribly small and out of the way on the Oasis. They purposely want to drive the passengers to bring their own and use the wi-fi.
RCI recently added a new 500 minute plan for $150, which sounds like what you'll need to buy.