A vertical indentation on the forehead that has been present since childhood can have several possible causes, including a congenital skin or soft-tissue difference, an old scar, or a localized change in the underlying tissue. Since you mentioned that it began as a mark and gradually became indented during your teenage years, I would recommend identifying the cause before choosing a cosmetic treatment. Restylane, a hyaluronic acid dermal filler, may help improve certain depressed areas by restoring volume beneath the skin. However, it is not automatically the best solution for every forehead indentation. If the depression is related to scar tissue, tethering, or an underlying structural condition, filler alone may not provide the desired result. The forehead also requires particular caution when injecting filler because of its blood supply and proximity to important blood vessels. Although uncommon, accidental injection into a blood vessel can cause serious complications, including skin damage and vision loss. Any filler treatment in this area should be considered only after a thorough assessment by a highly experienced, appropriately qualified medical injector. Other options may include scar-directed treatments, selected laser procedures for the pigmentation, or a combination of approaches, depending on the diagnosis. Because you have a deeper indentation along with hyperpigmentation, treating the contour and the discoloration separately may provide a better result than relying on filler alone. As for cost, it varies depending on the treatment, the amount of filler required, the provider's experience, and your location. Hyaluronic acid filler is generally priced per syringe, and more than one syringe or treatment session may be needed. Laser treatments and scar revision have separate costs. An exact estimate would require an examination and an individualized treatment plan. The encouraging part is that a long-standing indentation is not necessarily untreatable. The goal would be to make the depression and discoloration less noticeable while prioritizing safety and a natural-looking result. I would not recommend injecting filler into this area until the underlying cause has been properly evaluated.