I had no idea generators dedicated to producing AC power do it that poorly.
Years ago, I found this blog post so interesting. It dissects why the Apple iPhone Charger is so expensive. It comes down to the same reasons the Goal Zero might be more costly. It can filter dirty power into a steady output with voltage protection. You pay extra money for the longevity of the batteries you're charging.
Oddly though, their Yeti 400 is Pure Sine Wave
I wonder if that's why cost per Wh is more on the Yeti 400. For more than twice the Wh, I'd assume you'd get a price break per Wh not a hike.