If you don't have mobility issues I strongly advice you to take a train, it can take up to 2 hours but it is far cheaper than any transfer. One way tickets cost 5 EUR from Roma Termini to Civitavecchia. The only thing to consider is that the train station there is 15 minutes far (walking) from the ships. Taxis shouldn't be entitled to ask you more than 10 EUR for the transfer station-boarding point though.