What Is the Economic Cost of Mounting a Sustained 51% Attack on Bitcoin Today?

The cost is prohibitive, stemming from the immense capital expenditure for the necessary specialized mining hardware (ASICs) and the ongoing, enormous electricity costs. The attacker would also incur opportunity costs by diverting hash power away from honest mining.

The required hash rate is so vast that the cost of a sustained attack runs into billions of dollars, making it economically irrational.