£10 entry fee!

I don't know whether it's been mentioned before, but the Historic Houses Association is quite good value for us ( living in the York area ). I think it's just over £60 for a years membership for two people. Around here it gets us into Newby Hall, Castle Howard, Scampston (although not into the walled garden) and Burton Agnes, all of which have very good gardens, and also into many other interesting historic houses.
We haven't been to many outside Yorkshire (we did visit Abbotsbury Subtropical gardens for free a few years ago ) so I don't know if it's as good value in other areas but it would definitely be worth it if you did a bit of traveling around the country.