Don't know exactly what you mean here, maybe you are agreeing that all values are proportional to bitcoin value?
I'm disputing your claim that an attack becomes more attractive as the value of bitcoin increases. I'm pointing out the extra cost of the attack would cancel out the extra reward.
You did clarify later on that by 'valuable', you meant a more a steady/dependable price, and not necessarily a higher market price, so I guess in light of this my response is not that applicable to your point, and we can move on..
True, I did not take into account the initial acquisition value (which I would estimate at 10.000$ in the example above). I only took into account the depreciation of that hardware (and then added another $ per hour). I think that's not unreasonable and standard procedure in profit/loss calculations. The same trick could be pulled of multiple times with that very same hardware, or the hardware can be used for different purposes (e.g. video rendering) afterwards.
I don't think the attack could be pulled off multiple times. Either the value of bitcoin would plummet, or the network hashrate would increase significantly prevent future such attacks. The attack can't be a recurring source of income.