I would recommend that you consider the train variant. The cost is from 20 EUR per ticket, on the road about 4 hours, the trains are straight. Departure every few hours from Sants Station. More information can be found on the website www.renfe.es in English or Spanish

The bus runs about 7 hours, via Zaragoza, and its cost is from 25 EUR. On weekdays there are two trains from Santiago to Barcelona, ​​at 8:42 and 18:45, the first on the road is 12.23 and the second is 14 hours. On weekends - only the morning train. The morning train is non-stop. The price is about 45 EUR (depending on the date). You can also see the details on the website www.renfe.es.

But my recommendation to you is to replace this train with a plane. There are many domestic airlines, you can also see the neighboring towns of Vigo or La Coruña, getting to them is easy. The cost is from 30 EUR, and the summer is very little, around 2 hours with little.