Do you mean the valuation or the cost of the policies? The former has been rumored and repeated from various sources, the later would be a near impossible question... but first, here is a break down of what was rumored:
Senna supposedly carried a policy for $19 million on himself. Williams had a policy to cover the hit to the team should they lose their star driver, rumored to be valued at $6 million. Rothmans, who was the primary sponsor for Williams in 1994 also had a policy to protect against Senna's death, supposedly valued for another $12 million (all in US Dollars). It wouldn't suprise me if FOM or other related organization also carried policies, but I've never seen any mention of that.
As for the costs of such policies? That would be near impossible to answer. Even an educated guess by someone familar with underwriting such policies would likely be lacking key information to even come close.