They are so very much "it depends" that there's not much point. You have a million possible choices to make all of which will move the number around.
Very roughly for feasibility I would say the sale price should be four to five times the oneoff BOM cost, plus allocate $50k upfront for NRE including certifications. Experienced hardware developers can do better than that, but if you've not done one of these before you need to budget for errors.
I've done some of this on a small scale, and the number of potential surprises is just really high. Things like your aluminum frame being subject to US anti-dumping regulations. Or the supplier sending you power supplies with fake UL marks, and now the whole shipment gets seized...not just the power supplies, but the other stuff in the container too. Or your mold is off by just enough to make the already bought-and-paid for aluminum end pieces not fit. PCB oopsies that can't be jumpered around, etc. Or, one of your suppliers likes your idea and copies it and undercuts you on the finished products. Or that terrific, low-cost/high-quality supplier just disappears overnight, and you can't find a suitable substitute.
Hard to translate into numbers, but generally plan on some amount of throw-away work, get some experienced advice if you can, and grow slowly until you understand the risks.
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.
Very roughly for feasibility I would say the sale price should be four to five times the oneoff BOM cost, plus allocate $50k upfront for NRE including certifications. Experienced hardware developers can do better than that, but if you've not done one of these before you need to budget for errors.