Prices, at the most basic level, are the meeting points of agreement between sellers and buyers. Sellers try to earn as much money as possible, and buyers try to spend as little as possible. The range of where those two factors overlap is where one finds successful prices.
High prices are a symptom of low competition for products or services. Supply is low and/or demand is high. High prices of established products like insulin are, more specifically, symptoms of competition suppression. Market suppression can be caused by both government policy and private parties manipulating the system for protecting intellectual property.