I wonder if this has anything to do with how Hetzner operates in each region, they run their own EU datacenters but AFAIK they just rent rack space in the US, so they're more at the mercy of upstream providers.
The value of an exchange depends on the number of networks that are a part of it. The Amsterdam Internet Exchange (AMS-IX), Frankfurt Internet Exchange (DE-CIX), and the London Internet Exchange (LINX) are three of the largest exchanges in the world.
In Europe, and most other regions outside North America, these and other exchanges are generally run as non-profit collectives set up to benefit their member networks. In North America, while there are Internet exchanges, they are typically run by for-profit companies. The largest of these for-profit exchanges in North America are run by Equinix, a data center company, which uses exchanges in its facilities to increase the value of locating equipment there. Since they are run with a profit motive, pricing to join North American exchanges is typically higher than exchanges in the rest of the world.