(Question) Why do filler prices vary so widely?

My forehead is flat, so I am always self-conscious about the lack of volume whenever I take photos. I have heard that filler treatments are effective for adding volume to flat areas, so I am currently looking into various clinics to get forehead fillers. However, the prices vary wildly from clinic to clinic. Why is there such a significant price difference? What criteria should I use when choosing a clinic? Which filler products are best? As someone new to fillers, please let me know what I should focus on during the consultation.

(Photo source - Seomyeon New Face Clinic)