I'd say the cheapest way to reach Pisa from Volterra is by train. There's not a direct one but you can stop in Cecina with a train that leaves from Volterra and get on the one that'll take ya to Pisa from there. It's a an hour and a half trip. You didn't specify what day you are going to be there but you may want to give a look at the national railways transportation site. You can insert from where to where you wanna go and it'll show the trains you can do it with.

http://www.trenitalia.com/en/index.html