I just took a look at the Trenitalia web site and the current price is still 1.7 Euros (one way) for a 2nd class ticket. Of course, this could change at any time, but any increase would likely be minimal. A first class ticket (not worth it for a 15 min ride) is 2.6 Euros one-way. You must be careful looking at other third party sites since some of them will sell you a ticket on-line with an additional mark-up. There is absolutely no reason to buy a Pisa ticket (or Florence) in advance since reservations are not allowed on these trains (unless you were to book a more expensive IC train) and there are no assigned seats.