Budapest: 2.2 EUR for a nice Doner.
The best shawarma I was able to find costs 3 EUR in the innermost district.
Niedersachsen. I usually get a falafel tasche which is €3 (same size) but you're right, maybe the equivalent sized döner is 1 or 2€ more. Still, there's a large gap in price I can't get my head around. Maybe your tax evasion point could be it, or maybe the author's business is in an unusually high rent area (shopping centre or big city or something).
To add to this, the Dönnerbude is probably in a part of the city where rents are low (whereas US restaurants are often in designated city center high rent areas), and the Dönnerbude is usually very, very small.
I live in a cheap part of Germany, and our Döners have gone up from 4.50 or 4.70 Euros to about 5 Euros everywhere.
But let's not forget that even 5 Euros is an insane price for that much meat.
It works out because (a) the meat isn't great to begin with and (b) Dönerstuben are hotbeds for tax evasion. Many many Döners never make it into the cash register.