I like the idea(pay for permission), have thought of it before but also do not want to see the trend produce some unforeseen side effects that might pinch off any what is left of 'free range' detecting in the greater public world. What is good for some is not good for others.
As a side note, I also thought of offering services, say for yard work or odd jobs in exchange for permission to dig. At least this might show that you are not just about flashing a little money in hopes to get bigger riches from them(as if they were your spring board). You wouldn't be treating them like a lottery ticket machine because should they accept, they are getting some real and valued help, which I think renders a better public and personal standing between you and them(less the impersonalites of being able to buy people off). However, there are a lot of variables and negative potentials in that method too, say if you did a few things like removed brush and mowed a lawn, then they changed their mind and told you to leave...no money, no digs...of course, some form of contractual agreement has to be understood and put into good terms and standing at the onset.
I recently got to dig a nice big old house. Though I knew the owner from many years ago, and he is easy going and nice, I had originally intended only to buy a few items from him that he was selling. Thereafter, I worked the conversation into detecting...and he obliged...all this of course was after I forwarded the money for the bought items, but just the same had nothing to do with or was in any way in relevance to sweetening my position to dig there...but i figured, i would throw the idea in anyway. But to my surprise their curiosity invited me more so than I had to labor to 'ask permission'. I guess you can't seem too eager.
Permission granted digs are probably proportional to the act of swinging the coil itself, sometimes you get something and sometimes you don't. Let me know, anyone, if you find the permission elixir.