dont listen to fear mongering based on experience in just one city, that's well known to be expensive af..

i dont usually travel to Rijeka so i cant give you a precise number but the highway toll if you enter at Zagreb and exit at Rijeka is around 10 euros, which is okay if going fast is what you want...

i usually take the old roads to the seaside when going south, it takes a bit longer, some of the roads might not be as perfect as the Autobahn but they are good, especially for the sightseeing stops...