Wikipedia article for the Carrington Event
For the sake of the question, "another Carrington Event" is defined as a storm with a Dst of -1000 nT hitting the Earth.
This is estimating the cost to the US government to harden critical infrastructure and improve preparedness enough that a second Carrington Event would cause nothing worse than a severe economic depression, instead of societal collapse. As a rough operationalization of this: at least 95% of the people who were alive in the US at the moment of the event need to still be alive after 6 months have passed.
Assume the storm hits within the next 5 years.
The costs might include things like:
sun observation
broadcasting geomagnetic storm predictions
training AI to predict geomagnetic storms and their impacts
sunspot research
transformer design research
and whatever else accomplishes the goal of keeping 95% of the population alive for 6 months after the event.
The cost would not include things like the cost of political lobbying for the project to be funded in the first place.
This market won't resolve unless the event in question actually occurs (see @robm's comment for issues with this). The phrase "within the next 5 years" should be interpreted relative to when you are reading this, rather than relative to when the market was begun.
Related:
1,000