It made more than 2k for me per turn at the end of my Julii campaign. Caralis is Arretium's best export destination while also being one of the better export destinations of both Segesta and Massilia.

I'd garrison it with peasants and try to sink the Carthaginian ships. You can manuever the other Roman factions into helping you. It helps immensely mid to late game in naval superiority battles. You can retrain and build ships there. If you want to sell it, sell it to somebody you can easily make war on when you are prepared to keep it.