Most of my lights have drivers I built myself (through Oshpark & BLF threads), the 12 I have done so far are FET linear drivers and very simple. Even though they are simple the BOM alone came to 4+ USD per driver, which is high if you consider the few parts and functionality. I learned a lot from BLF and those drivers, but also spent more than 50$ in parts alone (excluding tools).
Other, more complex drivers have very high costs but can also be very good. Loneoceans had a driver design I really wanted to replicate, but the costs didn’t allow it.