Wikipedia has an interesting definition of “nominal fee”. 

“A nominal fee refers to a sale of a good or service for far less than it is actually worth, when it cannot be given away for free. Typically, such a sale will be for the smallest full denomination of a currency (for example, one dollar).[1] Nominal fees are necessary when contracts are signed, since in order for a contract to be valid, some form of payment must be involved.[1]” 

https://en.wikipedia.org/wiki/Nominal_fee