I think it will be based off high alchemy price. I was worried as well because a few lures rely on the alchemy price being the basis for the "kept item", but after I have read and re-read that Q&A, they state that it will be based on the highest valued item or highest worth/price. They never mentioned their newly coined term "guide price". If they planned on using the constantly fluctuating guide price that will replace the old mid-price, I would think they would say guide price when they talk about kept items.
I could be wrong, but that's the most i could gather to determine.