Interesting indeed. I'd try to calculate that but it depends on how many months were left in Kane's contract.
Even so, like in any insurance, it all depends on the levels of coverage and premium agreed for the specific insurance plan, and these can vary widely. I'd also guess that the law varies by country and league.
It essentially covers (fully or partly, again it depends) losses suffered by the club caused by the early retirement: player salaries (in their turn, negotiated in the player's contract), sponsorships, incidentals, etc.
Good post!