If you have a good handy man to do it for you then you can buy the ali floor kit direct from IW. Guessing around the £300 mark. To have a trailer centre do it for you possibly around £500
Shop around for this. My 505's floor went last year and also a side panel. My local IW dealer quoted me a ballpark figure of £1100 for the work, which had me reeling in horror. A friend suggested I phone his local dealer in South Wales, they did the same work for £690 (including a service), so well worth a couple of days out travelling and 2 tanks of petrol. I think the floor cost £350, there was a contribution from Ifor Williams towards it, for some reason I know not what.