return getBaseAmount().add(getAmountPerUnit().multiply(new BigDecimal(getNumberOfUnits(event))));
return getBaseAmount().add(getAmountPerUnit().multiply(new BigDecimal(getNumberOfUnits(event)))) .subtract(event.calculatePayedAmount());
    public BigDecimal calculateTotalAmountToPay(Event event, DateTime when) {        return getBaseAmount().add(getAmountPerUnit().multiply(new BigDecimal(getNumberOfUnits(event))));    }