The more expensive the cruise cabin the higher the OBC you can get. I've seen 1000OBC offered before for suites.
Also there are many ways to get OBC. If you own stock in the cruiseline you get credit, future cruise deposits on celebrity for instance of $100 give you $300 OBC for cruises 12 days or longer, sometimes the cruiselines will add credit rather than lower a price, or you can get it through a travel agent. Many times you can get several of these and they will add up.
Usually though I find a cheap base fare to be more important in getting a good deal than how much credit you get.
11 cruises in 2012, 90 nights at sea visiting 15 countries
8 cruises in 2013, 56 nights at sea visiting 7 countries
Next cruise: None scheduled
PM me if you need a cruisemate for any Europe, transatlantic, or Panama Canal cruises
Date, ship, cost pp