UK - 2500$ for 2 weeks, including air tickets. Depends how low budget can you go&find it could fit the bill hardly, in my opinion!

Croatia in May usually is not too god for swimming. Same goes for Greece islands (most of them open up to the public, I mean hotels, restaurants after 15th of May). Price wise, Athens is a bit of middle-high season, while the islands are lowest fares possible!

Athens and other destinations in Greece/Croatia are perfect in May!