@Nissanka1 Sorry, I had not seen your answer before.
It doesn't cost a lot, it actually costs whatever you want. It works more or less like a credit card. You buy the card and then you deposit as much money as you want. And if you don't lose it that serves for all your life. Without it each travel costs 1'40€ as I told you, but with they cost around 0'60€ if I'm not mistaken.
But that's only for Seville. In other cities buses work differently.