First of all:
Do NOT take a taxi, they're an expensive rip off, you're even better off taking an expensive high speed train... A taxi ride from R'dam to A'dam will cost between 145 and 180 euros.
R'dam center to a'dam center is more than an hour drive. Taxi's charge anywhere between 1-3 euros per km... 2 euros per km is ''cheap''.
Not sure where/what port terminal is, do you mean the Rotterdam cruise terminal ?
If it is ''Cruise Port Rotterdam B.V.'' (Wilhelminakade 909 3072AP Rotterdam ), then it depends how much of a hurry you're in, there's a metro which will take you to central ( you can also walk if you don't mind, it's a 20-25 minute walk at a guess, but it's the city center once you're over the bridge so it's not an awkward route), subway trains run every 3 minutes iirc ( I often take the metro in Rotterdam but not that line, been a year or so since I used it, you'll wait 5 minutes TOPS in the worts case though during the day). You can also take a tram to central but I wouldn't recommend it, the metro is quicker and both are priced the same.
From central you can either take a normal train ( +- 60 minutes travel to a'dam central) or a high speed train ( 25-30 minutes to A'dam), obviously the normal train is cheaper.
If you know the time of arrival + the date and the place ( eg. confirm it's the cruise terminal) you can plan your trip using public transport on this site ( address needs to be known though obviously where you arrive and want to go to):
http://journeyplanner.9292.nl/
For metro/Buses in Rotterdam you need a chipcard for every person, you can buy an anonymous one and fill it with credit or you can buy a 2 hour disposable one ( I think they're 3 or 4 euro's, there's others too like a day card etc..) from ticket dispensers at metro stations.
You can travel with the train using the anonymous card too, not with the disposables, but you can also buy regular tickets at the stations. The machines are also in english and they accept maestro debit cards or cash...
Ticket info:
http://www.journeyplanner.9292.nl/784/Ticket_pricing_home/
For getting around in Rotterdam ( so from the terminal to central station):
http://www.ret.nl/?sc_lang=en
For train to a'dam:
http://www.ns.nl/cs/Satellite/ns2007/en/mainmenu/include/1193053691834/travelers?p=1193053691834
For getting around in Amsterdam itself:
http://www.gvb.nl/english/Pages/default.aspx
But
http://journeyplanner.9292.nl/ bundles all the public transport companies together on 1 site when planning the trip, I'd use that... Plan the trip a week in advance ( not before though) and the timings should be spot on from that site.
Make that 50 miles in a straight line... Longer by rail and road...