How valuable is your land ? - Public forum - Planning Advisory Service (PAS)

Please help me out. I'm trying to test a theory that planning costs (ie the amount that an LPA spends on "planning") varies with how much developable land is worth. There must be lots of stuff about the economics of development, but I'm after a really simple measure - ideally a single number per LPA area. Can anyone suggest a suitable (freely available) dataset ? I thought there was something suitable in the old NI indicators but I may have just made this up.