‘Cost’ is applicable, because this money is money that has factored into the budget and public soending every year. To make up for not having it, you would need to find or cut the same amount of money elsewhere.

For what its worth, the reason that Tories want to cut this is because it disproportionately taxes the wealthy, and specifically generational wealth. Assets under £375,000 (iirc) are not taxed on inheritance

Of course tax billionaires and business properly, but cutting inheritance tax will increase the wealth gap and cost the goverment money at a time when vital public services are still suffering from 13 ongoing years of austerity