If these photos have whetted your appetite, you can get more information here clickylink .
I can tell you that golf club or spa membership is £1,600 a year and a Christmas 4 day House Party is £995.
If you just want to look round the gardens, they are open on 14th June in aid of the National Garden Scheme - £5 each though. Much better to 'go to something', then you get to look round the gardens free of charge.
Having said that, I don't know what's to stop you driving in, parking and wandering around - other than a great number of CCTV cameras I suspect?