Don't be short-sighted, a regional levy although the neatest theoretical option would be harder to implement and Gauteng already subsidies most of the rest of the country, so not really realistic (they can argue that all their funds should be ring-fenced). Just think about unintended consequences like anyone near a provincial border and how many business will have to close or relocate (petrol stations and the associated shops) due to this alone (crossing over the line to fill up at a lower rate).
Wow you don't pay attention at all do you? Chapman's was going to be closed, it cost more than half of the entire WC road bill. There is a main route via Ou Kaapse weg, Chappies is actually an alternative, very dangerous route. Therefore WC gov decided someone else can foot the bill for a tourist attraction. It not exactly a major thoroughfare now is it? You really want to compare a road with 20 users a day to the Gauteng freeways? Chappies closed = no affect on anyone getting to work etc, Gauteng freeways closed can you say the same?
What we really need on top of this is more transparency, publish EVERYTHING - all financials of all government agencies and parastatals. What on earth do they need to keep away from the public anyway? then let others analyse it and make up their minds on if something untoward is going on.
While GP does subsidize most of the country, they don't subsidize the WC. And we shouldn't pay for their local roads. There would be some side effects on petrol stations on the borders, but traditionally the effect isn't massive.