There should be no entry fee to get in to Greece, as its in the EU and you are an EU citizen (I presume) you shouldn't have to pay to get in. There is a departure tax to leave Turkey I think, and in total it should you about £30-£35 each for a return trip to Rhodes including all taxes and transfers and ferry. Check on
ICR website maybe for a up to date price but think thats what we paid and didnt have to pay anything else on top of that.