The are 5 different ways across 2 unique modes of transportation to help you travel from Zaragoza to Barcelona. If you're only considering the price, you should check out the bus out of Zaragoza towards Barcelona, as it is the cheapest to Barcelona. It's common to find tickets for as low as $10. ALSA operates from Zaragoza to Barcelona, so you book your trip with them! Book as soon as you can, as there may be limited availability.
On your trip from Zaragoza to Barcelona, it is best to book as soon as possible. It won't necessarily help reduce your price by a large amount, but will ensure that you have a spot on the bus or train.