We visited Alnwick Gardens today as a family of 4. It cost £13.75 per adult + £3.50 parking, kids were free as under 4, so around £30.
My opinion is it's a big field with a large fountain in the middle and a garden at the top. That's pretty much it for £13.75 entry. I just don't know how they can justify the price. I guess it depends on the time of year, but there were no flowering plants in the garden at the top either.
We visited for Halloween as we had heard there was some things on there, but this just seemed to be a couple of people dressed up as ghosts for 15 mins or so.
On the plus side the poison garden tour (15mins) was interesting, but it was just not enough to justify the expense.