Well, since you seem to know everything about it, lets assume you do, and look at how much your proposed system would cost...
Lets see what GPUs have ever shipped with 32GB of RAM...
the 5090...
oh... that's it. So... absolutely no "old GPUs with 32GB of RAM" are available.
Well, lets assume you meant 24GB for the sake of it, what GPUs shipped with that?
The 4090, the 3090, the A5000, the 4500, and the 7900. We're going to need 6 of them to get past 128GB. Lets be kind and call it 5, because the mac is going to use *some* of its 128GB for the CPU. How much do they cost?
- The 4090 with 24GB of RAM... $4500 on eBay, so that's $22500 in GPUs.
- The 3090 with 24GB of RAM... $1500 on eBay, so that's $7500 in GPUs.
- The A5000... $2500 each on eBay, so that's $12500 in GPUs.
- The 4500 with 24GB... $2200 on eBay, so that's $11000 in GPUs.
- The 7900 with 24GB.... $1200 on eBay, so that's $6000 in GPUs.
So... nope... it's not cheeper to do it that way. You litterally can't even buy the GPUs for that much, let alone the motherboard capable of running 5 GPUs with reasonable amounts of PCIe bandwidth each.