I got this idea from GTA IV
When A Player Cross City Borders or go from one city to another, My suggestion is that SA Govt. should deduct a Small amount of $500 or $1000 as Toll Tax.
The Tax should be valid for 24 game hours i.e a player has to pay Toll Tax once per in-game day.
Toll Plaza can be added to the roads, bridges connecting cities.
Example: