Rule of thumb in manufacturing is 5x BOM costs. This comment will be downvoted but only by people who have not been associated with a successful manufacturer.
But does that work for putting together what are essentially modular pre-assembled boards? The RF detector, display, charge controller, and ESP are already consumer parts, and they are put together not unlike how a desktop computer can be built from retail parts. Would you say that 5x retail cost for the parts is fair for a custom built PC?
The custom PC components are already finished goods by the time they get to your PC assembly person. Most of the markup is already baked in. I would in fact expect to pay $120 or so to have a PC assembled for me, so we still get to the $150 figure in either scenario. I assume you have never worked for, or originated, any such business. That $150 has to cover overhead like assembly labor, rent, credit card charges, insurance, accounting, bookkeeping, excise tax, chargebacks and refunds, advertising, website development and social media, bookkeeping, software subscriptions, fulfillment, compliance, and so on. Running any business in the western world is complicated and expensive these days.
If you’re imagining you can take $30 worth of parts, sell the finished item for $60 or $90, and sustain that enterprise, it’s time to reconsider your business acumen.
> "I have had personal experience trying (and failing) to drive down retail cost of electronics below this rule of thumb"
Didn't have to make such a statement because I paid attention to people who knew what they were doing. If someone had been this "insulting" with my attempt to beat Craigslist, OTOH, I'd have saved $1.4 million of my own money. I'm happy to be "insulted" by people with more experience.