At the rate agreed upon by the purchaser and insurance provider. The considerations are different than those of insuring gold or dollars, but to an actuary, they're all just probabilities.
I think the question was more, with whom does one ensure Bitcoins? Has any insurance provider stepped up and said that they have a solid-enough picture of Bitcoin's risks to know what premium to charge?
Many of the big web-wallet providers, exchanges, and other custodians insure their customers' Bitcoin holdings against theft. Their insurers include Aon (who insure Coinbase), Lloyd's (Elliptic), Marsh (Circle), and XL Catlin (BitGo).
