Probably costa or MSC may have some 7 night deals within this price bracket.
Doubt if you will get anything worthwhile at this price...........if you do it will probably be right at the bottom end of the market.
Thomson have fly cruises within this bracket but not in the prime holiday months.
MSC springs to mind, but remember you mostly get what you pay for.
Try Royal Carribbean. They have deals to the Med for less than £600 this summer and one to Egypt (Alexandria) on 1st June, inside cabin around £700. When would you like to go? this would have a bearing on the price and availability. Have a look at the deals on the home page of this site ...some really good offers if you are not too particular about dates or can go at short notice.
Thanks for your answer i was considering august or early sept but will make a few enquires at my local travel agent nearer the time so i might get a last minute deal .