There is a very small time between deciding that the price is at its lowest & it disappearing from the papers/teletext/internet. Also, a price that may look good can mean that the cruise company's cabin availability is limited & more importantly that the cruise company's original flight arrangements are all gone, so you are routed convolutedly to the embarkation port. Our 'rule-of-thumb is not to consider any cruise unless it is half the brochure price, below that & you are running the risk of the above. best 2 places we have found to get a great idea of cruise prices is Saturday's Daily Mail & teletext page 118. Scan these for a while before your planned dates, & you will get a very good idea about what/where to/what ship is on offer. Be careful of companies who offer a ship's cruise & add-on their own flights, these must contain a pre-cruise stay in a hotel, just in case there is a flight delay.
There are very few "BOGOF" cruise offers. They do occur from time to time but the best thing that you can do is to get to know what is a bargain price and what is a "come-on" There are times when cruise lines need to fill up ships because of underbooking or maybe even a change of itinerary and that is generally when you find the real bargains. Don't just believe that 1/2 price or 33% off is a is a give away until you are aware of what most shrewd people would expect to pay for that cruise.