This is a recent regression. If in the iOS Region settings you select a country with Euros as a currency (most of our customers) you should get a Euro symbol when expressing a currency with .ToString("c") on a numeric value. In recent versions of Xamarin.iOS (don't ask me exactly when) this appears to be failing, and a dollar sign is being shown which is useless and wrong.
Build environment is VS for Mac 7.3.3, Xamarin.iOS 11.6.1.4.
Comments appreciated.