The draft chart has some value in giving a rough guide, but it always depends on the circumstances e.g. the willingness to move down, how many competitors to trade up there are etc. QBs also always increase the cost. Ultimately it would take a boatload to move from 15 to 1 as Jax would be giving up a potential franchise QB to move down to a place where they likely couldn't pick a blue chipper. You are talking at least two future first rounders, two seconds, and Gilmore, and that's still probably not enough.
Even getting to Cincy's 5 pick is gonna be expensive. If you go by the principle that a pick next year is worth a round lower, then it's going to at least take the following (the trade chart gap is 650 points): Pick 15 (1050 points), pick 46 (440 points), next years second (220 points). But we know that given it would be for a QB and there are going to be many suitors, next years 1st would likely have to be part of the deal. The Jets for example traded 6, 37, 49 and a future second just to move from 6 to 3 in the 2017 draft.
You just get the sense that this isn't really part of Bill's philosophy, although he might be willing to change if he really thinks one of these guys is a true franchise player.