This column is about profit margins. Stay with me.

According to the Office for National Statistics, profit margins for non-financial companies are about 12%. In manufacturing, it’s 5.5%. So if we buy a sheet of metal for £100, the metal maker gets £5.50 profit for his sweat. Fair enough.

At supermarkets, it’s lower than that. When Tesco has a good year, which it hasn’t for a while, it gets £4 profit on our £100 of groceries. Practically a charity. Real-estate firms have a 16% profit margin. Dentists make less than you might think — 15%.

The profit margin at financial services company Hargreaves Lansdown (HL) is more than 70%. Its half-year results show it made a profit of £131 million on revenue of just £185 million, an astonishing return.

In any other industry, this would lead to howls of outrage and calls for competition investigations. For some reason, the City is mum on profits in the wealth management sector.

I ask Alan Miller at SCM Private, a campaigner for fund fee transparency, what to make of HL’s margins.

Wedging his tongue in his cheek, he replies: “It shows that they are incredibly efficient.”

It does. There is no denying HL is a brilliant business, which has gone from a Bristol bedroom in 1981 to a FTSE 100 giant valued at £6.5 billion.

Founders Peter Hargreaves and Stephen Lansdown — major shareholders but no longer involved in the day-to-day — have built a company that excels at attracting and keeping investors.

Our chart on the right shows just how loyal HL’s customers are — the money it receives is awfully sticky. Perhaps that’s because customers are impressed by HL’s undeniably slick service. Or perhaps they just don’t quite get how much they are paying for it.

Let’s say the fund you buy from HL makes a return of 4% a year — about typical in the long run. You lose nearly 1% of that to the fund manager, and another 0.4% to HL. Then there are trading costs. So you’re losing basically half your own profit.

This explains why so many customers of the fund management industry in general struggle to tally the story on the news about soaring markets with the pitiful returns on their equity savings.

The big problem with HL (although not for HL) is it charges on a percentage basis rather than flat fees.

It’s hard to see how this is justified any longer, if ever it was. Administering savings of £1 million is not obviously any more onerous than managing £50,000, but a 0.4% flat charge for either amount of money looks awfully expensive on the bigger fund.

If the customers could see what they were paying in pounds and pence rather than as a percentage fee, perhaps they’d object.

Coming this way soon is MiFID II, a far-reaching shake-up of many elements of financial regulation that will emerge when Brussels finally gets around to it.

Transparency on costs in the financial sector is part of it, and a good thing too. If clients saw what they were paying HL in pounds, perhaps those funds would find new, cheaper homes.

Perhaps the firm should get ahead of the curve and shake up its fee structure before regulators impose something worse.

HL says it “works tirelessly to maintain and improve the lot of retail investors”. If so, there’s a clear direction for it to travel just now.

Asked about all this, HL director Ian Hunter tells me: “Hargreaves Lansdown uses the influence and combined purchasing power of 876,000 investors to drive down the cost of investment. Last year, we saved investors £17 million in investment charges.” Is that all?

There’s no doubt that Messrs Hargreaves and Lansdown are among our most brilliant entrepreneurs, stacking up personal fortunes of £2.2 billion and £1.5 billion.

That’s couldn’t-care-less money — enough to buy private islands, yachts and football clubs. It’s money enough to start giving some back to those loyal savers, and make it about wealth generation rather than wealth management.

For those customers, of course, owning football clubs or yachts remains an elusive dream.