There is a difference between the best value vs the lowest price. A billboard on a road with a low traffic count would be cheap but would also have a low value. A billboard on a major road would cost more but the value would be higher because more people would see your ad’s message.
For digital marketing, the objective you set for your ads will drive the price up or down. The Impressions/Views objective is the cheapest way for your brand to be seen. But, that might not be the best value in terms of driving sales for your business.