Peppercorn sum

This phrase I think means a small or insignificant amount of money in respect to the item being referenced. Any idea on the etymology?

Literally what it says. Back in the old days pepper was very expensive. Small sums could be paid in peppercorns. As these became progressively cheaper, then so did the value paid in this currency until, today, it means that only a token sum is paid. It's often used in the form 'peppercorn rent'.