>Is this an example of a domain service in DDD terminolgy?

I would say yes.

>Should it be in a class library named: Core? (the Core of the application).

I don't know what kind of library is Core. But it should be right along all other Entities or Services.

Thing that bothers me tho is name. `CurrencyCalculator` is too generic name. I would name it `CurrenciesDenomination`, so it is clear it is calculating denominations. And I would put it in `Currencies` "module" in the domain library. Most probably right next to `ICurrency` interface.

And it is true that the method could be turned into static method with `ICurrency` and `cost` parameters. And it could be turned into extension method. But it would still belong in domain in `Currencies` "module". It would NOT be an `utility`, whetever that means.