You can bring back as much as you like provided its for your own personal use, however, there are guidelines as to how much is considered personal use.
Assuming you are flying into Barcelona and not Girona or Reus, taxi takes about 30 mins for �30, aerobus takes about 40 mins to Placa de Catalunya and I think is �4.50, or there is a train, but I don't know the price on that.
Can't remember the train fare from BCN into the city - it's a couple of years since I've been - but it was very reasonable and easy; pretty well straight out of Arrivals and on to the platform. It takes about 25 minutes to the Placa de Catalunya, if I remember rightly. Beware, though - the train gets very crowded and is notorious for pickpockets and sneak thieves.