On the one hand, low returns on bank robbery are fairly easy to predict, knowing that bank robberies are fairly rare.
how many banks are robbed? Figures for the UK show that there were 106 bank robberies or attempted robberies in 2007... In the same year there were 80 000 robberies recorded by the police, of which 7500 were committed against businessesOnly about 0.015% of robberies are committed against banks. If the returns were higher, presumably more criminals would be trying bank heists.
How much can bank robbers expect to escape with?
the average haul from a bank raid in the UK between 2005 and 2008 is £20,330.50. The standard deviation is £53,510.20. The sample size of 364 raids includes those that were foiled.... one-third of raids were unsuccessful.
Hauls in the U.S. are much lower, at an average of $4330.
On average there are 1.6 robbers involved (although in 60% of cases, there was only a single bank robber) allowing a quick calculation of what the expected return for each bank robber would be... and it's not a pretty picture for the criminals.
A longer summary of the paper's findings can be found here although the journal article is very readable in itself.