I just used the RW Events search for Half-Maras, specifying "South" and then "North" as region, the first ten I looked at in each case had unaffiliated/advance entry fees as follows:
South: 19,22,14,20,22,23,18,24,18,24 = 20.4 average
North: 22,16,15,20,28,22,15,28,26,16 = 20.8 average
so, more expensive in the North 