In Croatia, you have 2 options: Pay the toll at every single tollgate or, only if you’re regular user, you can purchase the on-board unit to obtain a 13 to 21% discounts on tolls.
The on-board unit is called ENC and you can purchase it in various points of sale. To calculate, if it’s more favorable for you, it is possible to use an interactive map provided by official institution of Croatia.
A1 Zagreb - Zadar (253 km): 121 kn (16 EUR)
A1, A6 Zagreb - Rijeka (135 km): 70 kn (9 EUR)
A2, A1 Macelij SLO/HR - Split (444 km): 181 kn (24 EUR)
A3 Zagreb Istok - Lipovac HR/SR (258 km): 128 kn (17 EUR)
These prices apply to the whole section of the motorway. If you travel shorter route, the price is of course lower.
The interactive map also contains the locations of points of sale of the on-board unit.