Ispog's largest (read 'only') bank was operated by David.
It regularly accumulated hordes of money through interest on its investments. (That is, small change was accumulated by David investing enough interest in the ground in order to locate and pick up lost coins lying there.)
Many customers (well, Stephen Temple, anyway) were caught out by the bank's punitive rules (eg. if you closed your account without first explicitly withdrawing all the remaining money, the bank (i.e. David) got all the money).
See also: The School Bonk