2 of 2 added 288 characters in body

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.