Cranioplasty is repair of a defect in the vault of the skull. This repair can be carried out by using bone removed at earlier surgery that has been preserved or by using bone from elsewhere as a graft. The surgeon will decided on the source of the graft. The iliac bone bounding the pelvis, ribs and even a part of adjacent skull bone can be used.
It is also possible to repair a gap in the skull by using synthetic material. Metal such as tantalum, silastic and other similar material manufactured for the express purpose of implantation into the body can also be used.
It is usually necessary for the patient to be in hospital for a week or so.
The simplicity of the operation depends on the cause of the bone defect. If the underlying covering of the brain - the dura mater - is intact and healthy, the operation is simple. (I assume that the overlying scalp is healthy and intact.)
The cost will vary from institution to institution. It is likely to be low in public sector university hospitals.