Delta IV cost per launch: $375 million.
Maximum payload: 31,250 lbs (6 tones)
Launch altitude: Geosynchronous orbit
Cost for storage container: $1.6 million
Weight of waste material: 12 tons
Remember that the rocket would cost a great deal more in order to provide boost to the sun. Also, please note that the rocket can dispose of only half of the container's material, making the equivalent cost well above $750 million as compared to $1.6 million.
Rockets are a lot more unreliable than a freight train or a barge, even man-qualified versions. Something falling from a near orbit is going to break open, vaporizing in the process. No such risk is incurred when these things travel by train.
31,250 lbs is closer to 15 tones than 6. We are not speaking about the past, we are looking forward. Rockets, in the future will be like trucks of the present. Someday, shortly, they will be able to launch from Earth, take their cargo for out in space, propel it toward the sun, and return to Earth for another load.
You mentioned a large cargo, in this day and age, plunging back to Earth causing a catastrophe...if precautions were properly taken, any failure would occur over the vast majority of the planet covered by oceans and nothing would happen. There is, somewhere under an ocean, a thermonuclear bomb lost from an American plane.....worried?????