Dent in my forehead, would it's appearance be improved by a filler or Botox? (photo)

From your photos the dent appears to be an anatomical concern and not to do with your glabellar muscles. Botox can actually make indentations and contours more prominent, so I would advise against the use of Botox. Filler would be your best option. Ensure you visit a highly skilled and board certified practitioner who has extensive experience in this technique. I believe you could also benefit from a combination of chemical peels and laser to improve the texture of the skin and encourage collagen production to fill out indentations.

From your picture, it looks like the "dent" is in the middle of your forehead and does not extend into your glabellar area(area between your brows). I think a filler would be best to be used in this area. Botox is injected into muscles and used to temporarily improve lines in your forehead (and other places on your face) so I would not suggest using Botox for your contour defect. The price of fillers varies depending on how much one uses and which type of filler is used. You should definitely see a Plastic Surgeon for an evaluation of what filler is best for you. 
Best wishes, 
Dr. Suzette Miranda 

This can be improved with a combination of well-placed fillers, lasers and microneedling/prp. I recommend getting a consultation with a facial sculpting expert. Best, Dr. Emer