Besides Rental Fees, What Other Costs Might an Attacker Incur during a 51% Attack?

Other costs include transaction fees for the double-spend transaction, potential losses from the coin's price dropping during the attack, and the cost of maintaining the secret mining operation. Furthermore, there is a risk of legal or regulatory repercussions if the attacker is identified.

The opportunity cost of not mining a more profitable coin during the attack also counts as an indirect cost.