Mine is 8' x 12' and was built with reclaimed panels (but still looks new) It was installed 25 years ago and cost just over £2,000 at the time. It's the compressor and fan unit that's going to add up. If you ask around and keep your eyes open you can sometimes find places that are ripping them out and then you can get them installed for the cost of the labour (weekenders) no guarantees of course but mine hasn't ginven me much bother in all these years and has only let me down once. If you're looking at 'all new' then you may struggle to get something a reasonable size for £4k