So obviously I'm doing something wrong at a basic fundamental level and could use a little assistance if someone's willing to.

The cause... we allow are players to opt out of being able to raid in exchange for being able to build in areas which are immune from raid damage.

The effect... some players are keeping their bases twig which is allowing them to build massive bases with very little upkeep cost.

The solution... simply increase the upkeep cost/decay rate of twig.

The problem... I can't make it work. 

Here's my config. I've tried many different settings attempting to see what might be the problem but currently the only changes from default are to not require permissions and the building grade multipliers. And no, I don't plan on using a x1000 multiplier on twig when I finally get it working. :)

{
  "General": {
    "Disable decay for all entities": false,
    "Exclude \"Other Multipliers\"": false,
    "Use permission": false,
    "Permission": "nodecay.use",
    "Output": {
      "Output to server console": false,
      "Log to file": true,
      "Log file name": "NoDecay-Log"
    },
    "Cupboard Settings": {
      "Disable No Decay if resources placed in TC": false,
      "Require Tool Cupboard": false,
      "Any authed on TC": false,
      "Cupboard Range": 30.0,
      "Block cupboard wood": false,
      "Block cupboard stone": false,
      "Block cupbard metal": false,
      "Block cupboard high quality": false
    }
  },
  "Building grade multipliers": {
    "Twig multiplier": 1000.0,
    "Wood multiplier": 1.0,
    "Stone multiplier": 1.0,
    "Sheet Metal multiplier": 1.0,
    "Armoured multiplier": 1.0
  },
  "Other multipliers": {}
}​

Okay, I think I just figured out my problem. I was under the impression that this plugin adjusted the upkeep cost but after looking at the code I see that it modifies the decay damage instead. My bad. :)