how much is a ria of land?

Answer 1

From wikipedia ...

"A ria is a submergent coastal landform, often known as a drowned valley or drowned river valley. Rias are almost always estuaries."

and (2nd link below)

"It is beyond dispute that the Crown is prima facie entitled to every part of the foreshore between high and low-water mark, and that a subject can only establish a title to any part of that foreshore, either by proving an express grant thereof from the Crown...."

... so, assuming the Queen is willing to sell, I would say, "really cheap" (seeing as it's only useable whilst the tide is out :-) )