Small rockwool cubes are very cheap, but the bigger ones can be a bit expensive. Is it possible to reuse the cubes? How is it possible to remove the old roots? What kind of cleaning is necessary?
No, you don't reuse rockwool cubes. You can't remove a plant from a rockwool cube anyway, so it should get planted with the plant. If (for any reason) the plants died in the cubes, you don't want to reuse them anyway, because they will be full of decomposing roots, which will often harbor disease, even if the original plants didn't. Allelopathy is also common, if the last plant had died quite recently.