Moronic says:
I'm surprised how little a replacement sealed bearing unit would cost
I think it is very likely that most bottom brackets owe all but a fraction of their retail price to branding and promotion rather than to development and materials and manufacturing cost.
Check out, for instance, the Stronglight JP400 sold by SJS for STG15:
https://www.sjscycles.co.uk/bottom-brackets/107-stronglight-jp400-plastic-cups-sealed-bearing-square-taper-bottom-bracket/?geoc=IE
They don't have it in your size but this mechanically superb bottom bracket is made by Kinex, bearing specialists in the Czech Republic, and branded for anyone who buys a big unit load, and sold around the world at surprising prices, low and high. It comes as a standard fitting on Eurobikes from the very top drawer, manufacturers who test every component compulsively. It's on permanent sale elsewhere in Britain for a tenner, and under other brand names than Stronglight has been under ten Euro on the Continent many times. It is made with a choice of plastic cups, as in the one SJS offers above, aluminium cups, or steel cups. The plastic cups aren't just for weight weenies: one manufacturer told me they fitted the plastic cups after their outside test engineer advised them that the small amount of movement the plastic cups permit is desirable for the longevity of the threads in the BB and for the exclusion of moisture because the fit is permanently tighter (presumably for the lifetime of the BB -- I didn't think to ask because these people don't sell anything you can't pass on to your grandchildren unless it is deliberately intended to be replaced at some specified interval). I was surprised to hear the high-buck, high-reputation names of some of the bottom brackets this single-euro (in wholesale bulk) BB beat out in a tough lab. I've had it on several bikes and it just soldiers on; I eventually went to the Shimano UN55 to get an axle length which suited a desired tread width, because Shimano stopped making the UN26, the previous Shimano value champ. A Kinex BB I took off and gave to someone else and fitted for him to ensure the job was done right is still working well after years of abuse like being parked outside in the rain for months.
(https://www.sjscycles.co.uk/images/products/medium/22940.jpg)
It's probably fair to say Kinex occupies the place Europe that Tange occupied for decades in the Far East, as the makers of everyone else's branded bottom brackets. The Tange BBs under their own brand that SJS stock in a fair size range are near the bottom of the page JohnR referenced.