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... and it will be very hard to prove either way (for either figure) whether the benefit is accurately priced. Also, once the govt's figure for the benefit of the open space is published, the builder will be able to massage his figures so they DO trump the govt's one. finally once a price is set it will stay there for years before it gets re-assessed, whereas all other prices will be rising - including the value of any potential benefit from a development.