Who ever came up with that price is looking for short-term profits over user happiness and long-term growth.
So, if you already have a sophisticated ad tech and sales team, you'd be able to pull 50%+ profit margin without having to worry about running the infrastructure for content.
That being said, there's maybe only a handful of companies with a more competent ad tech/sales team than Reddit, and Reddit's is pretty damn bad. So while the numbers make sense, the strategy does not given the competencies available in the market they're trying to sell in.