The banks pump out reams of numbers with their results. Other bankers inspect them, doubtless. City analysts take a lucky dip here and there to lend authority to their musings. But nowhere in this cornucopia of digits will you find what they spend publicising come-hither rates for savings accounts, let alone how much those accounts cost them.
Barclays are the latest I've noticed. They will pay you ten per cent on your regular savings, the posters shout - at a time when the Bank of England's official rate is anchored at 4.5 per cent -providing you pay at least £1,000 a month into a Barclays current account.
How do they do it? By spending shareholders' money. The cost is tucked away somewhere in the marketing budget along with the high-profile sports sponsorships no bank dare do without.
I suspect it costs less, too. Moneyfacts, an organisation that keeps track of these special deals along with more traditional banking terms, notes that there tends to be a
limit to how much you can pile into these special accounts, usually about £3,000.
That makes it less costly for the banks than those zero-rate transfers the credit card companies used to entice people who had run up big borrowings with a rival. They are having second thoughts about that now, discovering they have been outsmarted by so-called "rate tarts", nimble individuals who took care to move on to a new zero deal the moment the present one ran out. The card companies gambled on their forgetting -frequently wrongly, it now turns out.
This all started when Halifax merged with Bank of Scotland and found that while most of its branches were south of the border, most of its current accounts were north. The Halifax network dated back to its days as a building society when most customers had savings accounts and passbooks.
The struggle was to build up a critical mass of accounts for customers who had their salaries paid in to them direct. Halifax set about luring them in from the other big banks.
This was a tough job because once you have a current account, your bank has to do something pretty terrible to incite you to move it - the thought of shifting all the direct debits is daunting. It is meant to be automatic nowadays - but it is you shivering in a cold house if the debit that pays your gas bill gets lost.
Strangely, people who knuckle down to an evening of form-filling every year or two to switch to a new insurance company to cover their home or their car pale at the prospect of switching bank.
These eye-catching interest deals are intended either to change that - or to keep us that way. Some are defensive - HSBC launched an eight per cent savings deal for its existing customers last year just as Halifax launched a current account offering a one p er cent cashback on purchases.
Much of this is just marketing bally-hoo. But it is good to know that the banks value our custom so highly.