The price is set artificially. During congested periods it is set high to reduce demand for the regulated access based on the presumption that people will pay for faster transit.
So supply and demand is not setting the price, the operator is. But the operator is setting the price based on the idea that demand for the fixed supply (space in the regulated lanes) will vary based on the price charged for access. It's at least true at the limits, everyone will be willing to use the lanes if they cost $0 and (practically) no one if they cost $1000000.
The 'economic' behavior for the toll operator is to set a price that meets the various obligations set out for it while maximizing revenue.
Following the free-market analogy (for better or for worse), there needs to be an options market, where drivers can lock-in a price for the year, say $5 for that ($2 to $30) route. That way commuters can make better planning decisions. Then during periods when drivers don't exercise their right to take the express lane toll, Transurban (or the option-holders) can sell off their unused passes on-site.
As it stands now all of the volatility in transit times is externalized to the commuter.
So supply and demand is not setting the price, the operator is. But the operator is setting the price based on the idea that demand for the fixed supply (space in the regulated lanes) will vary based on the price charged for access. It's at least true at the limits, everyone will be willing to use the lanes if they cost $0 and (practically) no one if they cost $1000000.
The 'economic' behavior for the toll operator is to set a price that meets the various obligations set out for it while maximizing revenue.