Shockingly cheap. If I wanted boards this size in standard thickness (1.6mm), and would be ok with no ENIG, we’re talking $5 for 10-20 of them (total), plus $40 DHL.What does it cost to do a small run of custom PCB's these days?
These were quite a bit more (20 each design plus shipping for about $220) since they’re a very thin design (0.4mm driver 0.6mm LED), high-K aluminum dielectric, and ENIG. I could spend even more and do copper boards with direct thermal pads to the LEDs. These were from PCBWay, as well as the CNCed heatsinks
A lot of the cost is also NRE and special runs, which is why the cheapo boards are so cheap from JLCPCB (all the batching is already built in).
Ordering 10x as many boards the price per board drops massively (10x boards for 2-3x cost), up to a point.