How Does the Cost of a 51 Percent Attack Vary between Bitcoin and a Smaller Altcoin?

The cost difference is vast and is the core reason smaller altcoins are vulnerable. Bitcoin's enormous hashrate requires billions of dollars in specialized hardware and electricity to acquire 51 percent control, making a sustained attack economically infeasible.

A smaller altcoin, with a tiny fraction of Bitcoin's hashrate, can often be attacked for mere thousands of dollars per hour by renting hashrate. The lower the total network hashrate, the lower the barrier to entry for an attacker, making the coin a prime target.