you can't go by cost. If you look at the common magic item list for the last 4 codex's, you will find that costs on things differ. The item does not change, the cost for it does. It is about balance. If someone pays for a magic item, they expect to be able to use it. The fantasy rules have the dice off option for a reason. Mainly because there are holes and the rules are there for guidelines so the the game can be fun and fair for all. The last thing I want is to get stuck in a debate about who's more powerful. I thought what GW said was reasonable and it sounds like several people agreed. I am 4/4 of getting the same answer when I called, maybe the phrasing is the only reason I can think of that Bob is getting a different answer.
My phrasing was simple, when you have items in a challange that conflict, one being an item that says you are hit only on 6's and they opponent has an item that allows either auto-hit or hit on another number, how do you resolve it? I gave examples of the vampire lance and the silver lance auto hitting, the VC have an item that always hits on 2+, The flaming sword of rhuin spell always hits on 2+. All 4 times the answer was the same, roll off each turn to see which item works that turn. Jim's version still meets the intent of their ruling, just takes the randomness out of it. If you want to make up your own rules, that is fine too, but then you get into a lot of problems with people cheating and claiming that that is the way it has always been done here.
What it comes down to is either follow the rules or not, if you follow the flow chart on their website, we come back to the same thing which is to dice off for it. At the same time, don't let the rules be the nexus of your game.
In a recent game, my opponent failed to move his character out of charge range. He thought he had, but he was wrong. I declared a charge on the character, his answer was to pick up the figure and move it out of my charge. It upset me, but I did not stop the game because it felt like they were cheating, I adjusted my play and moved on.
The game needs to be about fairplay. Saying one item works and another doesn't because it cost less just isn't right.