It is just fair enough to have a parking fee, but to avoid confusion of some customers, why not implement at least a standard price per hour. It would be easy for customers & for the collector to compute unlike the ongoing fee which is .......2 riyals for the 1st  hour, 3 riyals each for the 2nd & 3rd hour. The 4th & the suceeding hours will be 10 riyals each.

1 hour = 2 riyals

2 hours = 5 riyals 

3 hours =  8 riyals

4 hours = 18 riyals

5 hours = 28 riyals

6 hours = 38 riyals & so on.....