Execution Plan Cost

At the backend, I'm measuring the cost of executing (via SPI_execute_plan) 
the read-only plan of a simple query with no reference to tables. E.g. 
simpleplan(int) AS SELECT $1 > 5

Executing this plan via SPI_execute takes around 70% more time than 
directly executing the relevant operator function (int4gt) and using 
DatumGetBool. Can anyone give me any insight into what operations account 
for the extra execution time?

Thanks,

Luis Vargas