In California, the law for public bridges and highways appears to be as follows (emphasis mine):
CA St & Hwy Code § 30843 (2017)
Any person who operates a motor vehicle over a toll bridge or toll highway crossing and the approaches thereto constructed or acquired by any bridge and highway district, at the entrance to which appropriate signs have been erected to notify traffic that it is entering upon a toll bridge crossing or its approaches and is subject to the payment of tolls beyond the sign, is guilty of a misdemeanor in each of the following cases:
(a) If the person refuses to pay the tolls.
(b) If the person turns, or attempts to turn, the vehicle around in the bridge, approach, or toll plaza where signs have been erected forbidding the turning.
(c) If the person refuses to pass through the toll gates after having come within the area where signs have been erected notifying traffic that it is entering the area where toll is collectible or where vehicles may not turn around and where vehicles are required to pass through the toll gates for the purpose of collecting tolls.
So it appears the "last exit before toll" signs are sufficient. It doesn't say anything about posting the actual cost of the tolls.