I'm a proud member of that club ?
I have been watching the progress of a couple of livaboards CCruiserswith 2 young kids who are slowly getting on with fitting out their 60' widebeam sailaway and I have to say that despite being 'skint' they're doing a super job using a combination of new and re-cycled products. Even with the kids on board they manage OK as they have all that extra space to work with. So despite the all negative reply's and examples the answer to 'the cost', has to be simply, (once you pay licences, moorings, insurance, fuel etc) as much as you want to, or can afford to pay.
Fittings on a wide beam can actually be cheaper than on a narrowboat as you can use 'normal' size beds and furniture but if using any fitted kitchen cabinets make sure you seal any MDF before fitting.