I really like Chelsea and it's good value compared to concerts and football matches cos you can stay 12 hours and go back to see favouriet exhibits more than once and at difefrent times of day with different light. It's busy but good natured and actually not that busy till about 11:30. I stay with friends so just have to pay for fuel on top.
I've also been to Hampton Court and hated the long long walk to the car park and the crowds. I've been to Tatton which I really enjoyed but it's a long way from Belgium so that's hotel costs on top and loads of fuel and, while it's fun buying plants, they're not cheap. The gardens are not as good as at Chelsea but I like the back-to-back section and it was great to see old boarding friends and meet Trillium and see her garden.
I've also done Malvern which is pretty disappointing for display gardens but great for plant buying and I have rellies I can call on if needs be.
The RHS also does monthly shows in London which are free to members and members get free entry to the big RHS gardens as well as quite a list of associated gardens all over the UK and some abroad. That plus a good magazine each month makes it exceptionally good value for the annual subscription..