This is pretty much the problem I'm having.
I can narrow the 68 puzzle down from.
295,147,905,179,352,825,856 Combinations to
402,120,015,509,709,217 Combinations
With a estimated 35-40% chance one will be right.
but even with 3 zeros removed its still requires a huge amount of compute.
I just assumed with only 0.15% of all possibilities needed it would take a regular 3090 like one week, but even with 8 3090s it would still take months.
I have no idea how people plan to pool mine this thing, will probably cost more in power than it is worth. unless you have a asic + low cost power and custom code.
if 8 3090s can check say 30billion keys a sec total (its not just a single 256 hash so prob cant even do that). it would take me about 140 days to check my guess.
so if you had 8000 3090 you could check every combo in 140 days estimate. From memory when I had my mining farm 60 Gpus were about $1000 a month in power but only using 100watts each, so your looking at about $200,000 a month in power or 1.2m to check every combo and also the loss in gpu value as tech devalues. Lets say 10% loss in value, or 800gpus, so like another 800k probably.
So for $2 million dollars you can crack this thing

Your numbers are way off. Even in your power costs for running 60 GPUs at 100w each, unless you live in a place where power costs are extremely high, which could be. At 15 cents per (kWh) costs, you'd be looking at $684 a month. For a $1000 a month, you'd be looking at 23.1 cents per kWh. That's pretty high IMO.
If you base the average solution time (the keys checked to actually find the solution) off of 67's, and you had to rent every GPU through a public supplier, you'd be looking at about $686,000 to solve 68. A lot less than the $2,000,000 you spoke of. If you owned your own GPUs, and had costs of 15 cents per kWh, you'd be looking at costs of around $206,000. These numbers are based off of using 256 x RTX 4090s, at 6.8 BK/s, which there are software versions out there that get over 7 BK/s, so I went a little lower.
2^67 * 57% / (6800000000 * 60 * 60 * 24 * 256) = 559 days
Unless, my numbers are way off, or I am not understanding what you are saying, both could be true lol.