Their Oyster-equivalent is a Navigo card. You can load a card with N single journeys in advance, up to 10 maybe. On the Eurostar train the buffet car will sell you a Navigo card preloaded with 2 journeys for (I think) Euro 7.85. I did this and it worked fine, my wife had a Navigo card left over from a previous visit. The alternative is to buy Navigo cards at Gard du Nord on arrival, but only some ticket machines will sell you a new Navigo (most machines will top up an existing one). I think that might be a slightly cheaper option but will take a bit more time. The website seat61.com has more detailed advice. Each time you use a Navigo a screen on the ticket gate tells you how many trips you have left. One warning: you need to pass through the barrier swiftly: my wife wasn't quite fast enough and her wheely bag which was trailing behind got briefly trapped by the rather enthusiastic gate.
Another warning: getting through security at St.Pancras was much swifter and easier than we expected but in contrast the Eurostar terminal at Gare du Nord was totally chaotic, with queues in all directions, egates for passports not working, awful baggage conveyor for x-raying baggage, and in general almost complete disfunction. Get there in plenty of time.
