Here's that same chart, focusing on 0-5k subscribers. For all those people with under 5,000 subscribers, Ghost is significantly more expensive.
It's $100/ year just to get started. If you make it to the magical 1,000 subscriber mark, your reward is...doubled hosting costs. If you make it to 3k you're pushing $500/year, and at 5k subscribers you're bumping up against $1k/year.
This is exactly the range where people struggle to monetize a growing newsletter.