We've got the technology to drill deep bore holes, kilometers below where water will move on any time relevant to human existence, where we could stick what small amounts of dangerous material is left.

Encapsulating the waste and burying it somewhere on an abyssal plain in the deep ocean should also be a pretty low-risk approach, the IAEA has looked into variations of that.

Like someone else said, I think it's all expensive because some people want it to be.