Is there any need to think of fungi? Doesn't the expression mean no more than that things may 'cost ' more than they might seem to, they are not, in reality, how they appear to the innocent ? A penny bun, meaning a bun, not a fungus resembling a penny bun, (or a penny whistle, come to that ) is called that but the real cost is higher ! The one pertinent example I found on the net was someone's mother saying to someone about to be wed " When you are married, a penny bun costs tuppence".. In other words what you innocently expect from description or imagination, but without experience, is not quite what you'll get. So be wise, because an expectation is likely to be misleading.