Unless social programs turn out to be unexpectedly cheap or gun law enforcement unexpectedly expensive, gun laws are probably the cheapest, smallest-delta intervention against mass shootings.
Gun control cost the Democratic party control of the Congress in 1994 and the Presidency in 2000. That's about as "expensive" as policies get in the US....
And as noted in this discussion, any attempt to squeeze tight enough that there might actually be an effect on mass shootings of the current type would replace them with fantastically more "mass" shootings of the civil war type, which tend to be only exceeded by foreign invasions as "expensive".
