How much should a taxi be? what other alternatives?
It's only 5 minutes by taxi, so it won't cost much. Or walk there in a little over 20 min.
Or walk from the port to the Municipio station, take the subway line 1 and get off at Museo - Cavour.
The big rocks near Municipio are old.
Naples taxi trips are largely fixed rates. Is this a cruise port day? Are you going to Pompeii too? Luggage?
Here is the taxi rate sheet:
https://www.napoliunplugged.com/tariffe_taxi_2017
It shows 11€ for your trip, including luggage. It is advisable to print out this sheet at home and carry it, since some drivers will try to overcharge you.
Or walk from the port to the Municipio station, take the subway line 1 and get off at Museo - Cavour.
This is what I did. The metro station is right next to the museum.
BTW while you're at the port get some Cannoli from this place https://goo.gl/maps/byXamcX7Z1x6KZTd8
Absolutely delicious!
We are considering the bus that picks you up at the port for the leg from the port to the ruins and the local train from the ruins to the port so that we can stay at the ruins longer than the return bus allows