The answer to your question is on the website I already gave you on your previous question if you bothered to look.
Just go to transport - it gives you taxi prices in euros to all the different parts of Barcelona. It is in Spanish but easily understood.
could be as low as 15-20, but if you hit a bit of traffic in the town centre (like I did) it'll be nearer to 30 9especially with a tip).
One other thing - be very careful with your wallet and bag.
street crime - pick pockets, bag stolen while distracted - is common - so take extra care, and have fun
The mainline train service (RENFE) runs a shuttle service every twenty minutes to the main station Sants, it's much cheaper than the taxi. From Sants you can board the Metro to pretty much any where, (or pay less for a much shorter taxi ride).
Barcelona taxis are unmistakable - black with bright yellow doors and boot-lids.