> There are DDR4 interposers you can buy for 50$.
Indeed, you can buy a piece of fiberglass shaped correctly for 50$. That's not the hard part. Just the probe you are supposed to connect to such a PCB is > 1k USD per pin you need to sample. The oscilloscope / logic analyzer to sample it is likely 6-7 figures.
> Or you can use an FPGA to interpose the RAM and intercept the network traffic for a couple hundred bucks.
What FPGA solution do you have for a couple hundred bucks could interpose DDR4 RAM at any frequency? This number seems completely made up to me.