Demand for DDR3 is up because people who want DDR5 or DDR4 but can't afford either any more are choosing DDR3 and old DDR3-compatible systems to put it in, instead of what they really want.

At the rate we're going, soon we're going to draw from SIMM stock.

Just to be clear, this was to go into an ancient Dell T420 NUMA system. Well over 10 years old.

All memory products use many shared resources in the supply chain, so if there is high demand in one product line, others have to raise prices to compete for the resources or stop making those lines altogether.

That is to say at least you were able to buy them at $350 today, with the current trajectory there will be no supply at all in few months.

You could set up swap space on Intel Optane media, it'll be about the same performance as DDR3 and sells for ~$1/GB on the secondary market. Though it will be a lot more power hungry than Flash, let alone DRAM - so not suitable for all uses.

Doesn’t that require an Optane capable system?

Optane is a technology I’m still mad never became mainstream. It would be particularly useful today when trying to run local models.

Optane is available in NVMe form factor that will work basically everywhere. There's also Optane persistent DIMMs that only work in highly specific systems.

there's an economic term for this: substitute good. https://en.wikipedia.org/wiki/Substitute_good

Being desperate?