They got Assange in a virtual prison for ~7 years at no cost to them (that cost was borne by the UK and the Metropolitan Police). You can't argue that being holed up in an embassy is that much better than prison.

He was "holed up" by choice. He was free to leave any time. Of course that would have been arrested and sent to Sweden for prosecution, but choosing to run and hide from an arrest warrant isn't the same as prison.