What can be done to fix my dent in my forehead? It was after an injury as a child. What cost am I expecting? (Photo)

Since you had previous trauma to your skull it is important to see  what is causing tthis area of depression. If an implant is placed it need to have inrowth into the implant so it doesn't move and also need to blend at the edges so it is not visible. 
There are many options of fillers including carbinated calcium phosphet cement, and mutiple comercially available curshed bone fillers, including bone pastes, and osteoconductive graft subsitutes. Fat grafing is another possibility. 
Neurosurgeons probably have the greatest experince in this area although may not have the greatest concern about the final aesthetic appearanceso also consult with a Board Certified Plastic Surgeion about this many options.

Hello Castro_Corona,There are a few ways to repair a forehead contour irregularity like yours.  The most straightforward and least invasive way involves making some incisions within your hairline and then putting a mesh implant to lift the tissue out of the dent and camouflage it.  More aggressive treatments involve a much larger incision and reconstructing the divot with bone grafts.  The latter option is not usually needed, especially if you haven't had any problems in 8 years.  I'd recommend you consult with a facial plastic surgeon to determine a specific plan for you.  I hope this helps and good luck.