One reason it is worrying is that, if you put a price on it in this way, as soon as a builder puts a higher 'benefit price' on his development it will be very hard to deny planning permission ...
Are we anticipating Barratts, Wimpeys, Persimmon or Bellways putting in a bid against ALL the open spaces in the UK at the same time, then. After all, all this report does is put a monetary value on the totality of open spaces and other natural areas within the UK. It won't help a specific builder or developer 'match' or 'beat' the value of a specific patch of land, any more than the existing land values list does. In fact, I would suggest that the breadth of the report - and its value - is that it highlights the value that such areas have not just for immediate financial benefit (as you seem to be suggesting) but indirect value such as for health. In view of the huge NHS budget, anything that reduces our reliance on this budget can easily be argued for against unnecessary/inappropriate development.
As such, I would suggest that the findings of this report will actually benefit those who are working to oppose such developments, rather than the developers themselves.