There are very few 7 day cruises on the Med. My suggestion is to hook up with one of the free or 2 for 1 air promotions like Oceania is offerring on some of their cruises. In some cases a 10 cruise with free or discounted air could be cheaper in total cost than a 7 day where you have to pick up the air fare.
We got a great deal on the Splendour of the Seas for August 28th. It's a 7 day Barcelona RT. There are still junior suites available (around 2400 per cabin). My parents' inside was under $1800 including insurance but not air, of course!
Costa also usually has some short Med cruises. I've been on Costa in the Caribbean and liked it but have not been on Costa in Europe. They have some really unique itineraries though. Just offering that as an alternative for you to check out because I don't think it's the best cruise line in Europe. My husband and I are really most interested in the ports in Europe, so it just depends on how important the ship's amenities (shows are not going to be as good on Costa, etc.) are to you.