It seems like you've been burned too many times with these issues. I'd be curious to know what you have built / brought to market and what you think about this rule of the retail price being 3 ton 4x the BOM.
Small/medium sized signage related stuff. For me, since I was low volume, a BOM rule of thumb wasn't super useful, as things like shipping from the manufacturer, breakage, reliability, etc, were just as big a driver and varied by product. BOM mattered more for small/light things, but I did variety in small quantities, so I don't have any useful rules of thumb.
One thing notable that I didn't predict well was competitors that would sprout up selling for way too low. They wouldn't last long, eventually disappearing, but hard to compete with that.
