The merchants who are considering using the tollway would be doing a cost-benefit analysis.

For each product that could be shipped via the toll route, merchants would be considering its purchase price, its sale price, and the cost of transport, whether that is via the tollway or via another route. They would also be considering risk vs reward.

If the cost of transport plus the tolls are less than the cost of transport avoiding the tolls, and profit over time is greater than otherwise, then the merchants will prefer to use the tollway.

The risk of loss of the cargo due to criminal activity when using the tollway - the probability of loss times the value of the cargo as part of the cost of transport - should be factored in as part of the cost of transport.

Obviously the purchase and sale prices of each product transported will be set by market forces. That leaves only the cost of transport and the amount of the toll. Since we know that the toll is 25% of the sale price, it stands to reason that unless the cost of transport (including potential losses due to theft) via non-tolled routes is greater than 25% of the final sale price, the tollway will fall into disuse. Merchants do not use tollways when they cut into their profits.

Of course, the goods being shipped via the tollway or other routes would need to be products that are only readily available on one side or the other. Goods that can be produced and consumed locally would not be shipped since they would not be profitable with such a large toll.

However, having large tolls is risky even for those imposing the tolls. The possibility exists that the excessive tolls will encourage smuggling and/or discourage trade entirely. If there are goods that are highly desirable and not produced locally on one side or the other, but aren't worth transporting, their availability will fall and their price will increase to the point at which they are once again profitable... and it is the consumers who will end up paying the price.

Finally, there is the risk that such greed on the part of the authorities who have set up the toll point will prompt the victims of these tolls to consider if the cost of the tolls is greater than the cost of hiring mercenaries to destroy the toll-point. If the financial and political cost of military action is lower than the tolls, don't expect this toll castle to survive very long.

The merchants who are considering using the tollway would be doing a cost-benefit analysis.

For each product that could be shipped via the toll route, merchants would be considering its purchase price, its sale price, and the cost of transport, whether that is via the tollway or via another route. They would also be considering risk vs reward.

If the cost of transport plus the tolls are less than the cost of transport avoiding the tolls, and profit over time is greater than otherwise, then the merchants will prefer to use the tollway.

The risk of loss of the cargo due to criminal activity when using the tollway - the probability of loss times the value of the cargo as part of the cost of transport - should be factored in as part of the cost of transport.

Obviously the purchase and sale prices of each product transported will be set by market forces. That leaves only the cost of transport and the amount of the toll. Since we know that the toll is 25% of the sale price, it stands to reason that unless the cost of transport (including potential losses due to theft) via non-tolled routes is greater than 25% of the final sale price, the tollway will fall into disuse. Merchants do not use tollways when they cut into their profits.

Of course, the goods being shipped via the tollway or other routes would need to be products that are only readily available on one side or the other. Goods that can be produced and consumed locally would not be shipped since they would not be profitable with such a large toll.

However, having large tolls is risky even for those imposing the tolls. The possibility exists that the excessive tolls will encourage smuggling and/or discourage trade entirely. If there are goods that are highly desirable and not produced locally on one side or the other, but aren't worth transporting, their availability will fall and their price will increase to the point at which they are once again profitable... and it is the consumers who will end up paying the price.

Finally, there is the risk that such greed on the part of the authorities who have set up the toll point will prompt the victims of these tolls to consider if the cost of the tolls is greater than the cost of hiring mercenaries to overthrow the toll-point. If the financial and political cost of military action is lower than the tolls, don't expect this toll castle to last very long in its current owners' hands.