If you consider flying you might want to check out Ryanair.com.
It's a very basic airline, but they normally have HUGE sales. Just a quick example that I looked up: a airfare between Rome Ciampino (which is actually closer to the city than FIU) and Barcelona in May was only 30 euro (this excludes a 10 euro booking fee and any baggage costs).
Not everyone fancies Ryanair (or their competitor easyJet) but, I have to say that I never had any problems flying with them. You just have to "fly by their rules".