Pricing on targeted ads is linked to confidence that a person will perform an action (buy, sign-up, etc). This is a function of the "richness" of data collected -- does the data about you contain demographics, engagement, behavior, so forth.

So a shoe company may pay a lot for a "male/18-35/basketball-fan/shoe-blog-reader". But Safari does not easily allow for that richness of data to be collected, so the price is low.