When crude prices fall, oil companies tend to pass on the reductions -- albeit grudgingly -- to motorists. In most of Africa, the same cannot be said of interest rates.
In the last two years, African central banks have slashed benchmark lending rates to try to pep up flagging growth but only in South Africa, far and away the continent's most sophisticated and transparent economy, have commercial banks responded by offering people and companies cheaper loans.
Of course, it is not a uniquely African problem.
After the collapse of Lehman Brothers in 2008, credit markets froze up across the world, with banks in financial hubs such as London and New York so nervous about potential borrowers going bust that they imposed sky-high interest rates.
But in frontier Africa, where more and more foreign investors are taking their first tentative steps, it is a lasting rather than temporary problem and one that highlights the inability of authorities to influence the cost of money.
In a nutshell, across most of frontier Africa nobody really cares what central bank interest rates are.
We report it even though deep down we know it doesn't matter, said one Johannesburg-based frontier Africa analyst at a leading international bank.
The lack of effective monetary policy transmission, as economists call it, might be expected in highly dollarised and war-scarred economies such as Democratic Republic of Congo or Angola.
But it is also notable by its absence in more developed locations such as Nigeria, Kenya. In Ghana official interest rates fell last year to 13.5 percent but the average lending rate charged by banks held above 25 percent.
The reasons are many and vary from country to country, but common themes are high levels of distrust between bankers and borrowers, and a lack of price transparency and competition in a sector whose main role for decades was simply bankrolling governments.
Banks also gripe about the high cost of raising funds in illiquid domestic capital markets.
A 2010 study by Kenya's central bank, which raised eyebrows in January by cutting its benchmark rate to 5.75 percent despite accelerating inflation, found that commercial banks ranked the repo rate eighth in a list of factors affecting their lending rates.
In Kenya, there just isn't the consumer pressure that you find in other markets such as South Africa, said Standard Bank economist Simon Freemantle, who used to be based in Nairobi.
You don't get people knocking down the doors of the banks saying 'Why aren't you cutting our mortgage rates?'
This is not to say central banks have given up on repo rates.
Zambia introduced an overnight lending rate at the end of 2009 as a first step away from monetary policy that had relied predominantly on money supply and the exchange rate.
Across the continent, authorities are trying to deepen capital markets, to make it cheaper and easier for banks to raise funds -- and thereby pass on those savings in cheaper loans.
And Kenya has introduced a Credit Reference Bureau to allow banks to check an individual's credit history.
But such moves take a long time to have any impact, meaning that for some years yet frontier African monetary policy will be dominated by blunt tools such as the exchange rate, reserve requirements and the printing press.