I never said all retail is over-priced actually only certain units. And yes DIY is not cheap which is a factual statement. You like to push DIY to people with "price calculations" yet in the real world those prices are never entirely accurate.
To realize any savings in DIY lights, I believe one needs to use the least expensive parts. Cree COBs, Epileds, and 1/16" angle aluminum instead of purpose built heat sinks. Instead of a Bluefish, use a TC421.
That said, I belive fixtures like the Twinstar use cheap leds, there is no inherent superiority to diodes they use. In fact, there isn't much difference between a Beamswork DA FSPEC and the Twinstar. They are both fixed RGBW, but (I think) the twinstar comes with a diffuser, and the white may be a bit warmer.
The problem with DIY isn't one of price, but of being able to recreate the desired effect.
My current DIY light is only using 12x Cree COBS (1.20/ea) and 26 Deep Red 660nm Epileds (.60/ea) on three LDD-L drivers. This system approximates the look of a Finnex Fugeray Planted+, provides 150+ PAR at the substrate, and could be built for $75, including TC421 and power supply.
Future DIY lights I build will be variations on this theme. High-K white + large# Deep Red diodes with or without controller. This look just works. Adding blues and greens might be able to fine tune the look, but getting the White/Red ratio correct gets you 95% of the way there.