E+/extra legroom Y on US carriers might just be that expensive, compared to European ones. Attached below I have placed what DL and AA would have charged if one would upgrade to extra legroom.
For European carriers, it does appear to be significantly cheaper to upgrade to an extra legroom seat, but the amount of these seats are much less as most planes don't even have a section, just an exit row usually. LH site shows starting from 66 Euro/$71.50 for extra legroom.