Is a 51% attack guaranteed to be more expensive than BTC gained by that attack?

51% attack is easier when there is a lower hashrate, which means that BTC is probably not worth mining at the time since it is not at a profitable price. However, this also means that the BTC is not worth as much to the perpetrators of a 51% attack. Would the attackers have to spend more money on equipment and electricity than the rewards they reaped from the stolen BTC? Billions of dollars in mining equipment and at least $1000000/hour in electricity costs. Additionally, if a 51% attack was successful faith would be lost in the network, and due to sell pressure the price would fall making the reward for stealing BTC even less. It seems like the only reason to perform a 51% attack would be to ruin the reputation of BTC but would cost the attackers far more than they gained. Am I correct in my assumptions and is there any way to quantify it?