For second hand gear RDIMMs are significantly cheaper than UDIMMs. As all RDIMMs have ECC that means generally ECC is cheaper for second hand systems.

Aliexpress had $50 single socket motherboards for Xeon E5-26xx CPUs and $150 dual socket motherboards last time I checked that use DDR4 RDIMMs.

Before the prices went up due to AI foolishness DDR4 RDIMMs were $1.20AU/G means means something like $US0.80/G.