It will take 5k 4090 GPUs * 114 days (total of 13.7m gpu hrs) to solve 69 puzzle which is 2^68 keys of work.
At cheap prices of 0.1 usd/gpu/hr it costs 1.37m usd.
I think 0.1 isn't cheap. And also you assume that a 4090 can only do 7 GK/s just because that's the fastest public SW. Anyway, 69 was too risky from the start.
What would you say is cheap price for 4090 renting in bulk say 1000 nos or more for 6 months? Also any guess on cracking rate of private repos on 4090 with super optimized cuda/ptx code?