My mother went there for tea and a scone with a friend last summer. It was €15, but for that she got 3 small scones (with butter, cream and jam) and a pot of tea. She felt it was quite good value given that you get to hang out in such beautiful surroundings. They were there for well over an hour and no one rushed them or made them feel like they needed to get out once they finished eating.
Now, it was raining the day they were there, maybe on fine days it was served outside, but I took a walk around the gardens when I was there, and to be honest, I wouldn't consider it a hardship to take my tea and scones out there on a sunny day!