There are usually quite cheap flights to Barcelona because it's served by 3 airports, (Barcelona El Prat, Reus and Girona) so there's plenty of competition among airlines. Central and East European destinations also tend to have cheap flights (because they're not as popular as 'sunshine resorts'). With luck you might be able to get return flights for under £100 but I'd budget £150 to £200 to be on the safe side.
Assuming a 'long weekend' to be from, say, Friday morning to Monday evening, that's just 3 nights accommodation needed and you shouldn't need to spend more than £50 per night. This website should help:
http://www.hostelworld.com/
(Don't be put off by the word 'hostel' in that URL. There are plenty of good, cheap hotels and B&Bs on there as well).
So, £150 + £150 = £300, giving you £500 left over to spend in 4 days. Bloody 'ell, just how much are you expecting to drink, BBB???
;-)