http://www.lloyds.com/~/media/lloyds/reports/emerging%20risk... was prepared by Lloyds of London after the 2012 detection of a large solar flare by a satellite in interplanetary space. (That one missed the Earth.)
Their estimate was $600 billion - $2 trillion dollars of damage from an event that happens roughly once ever 150 years on average. Therefore the amortized cost of this risk per year is $4 - $13 billion.
If a substantial fraction of the risk can be mitigated over the next decade for $300 million, it would be cheap at 10x the price. In fact Berkshire Hathaway probably has enough exposure to this risk that it makes financial sense for them to not debate over who pays and to just create the stockpile to reduce potential future insurance claims.