You can't judge the cost of signing Allen that way. First, the next year's draft pick is an unknown quantity, and is a big risk to trade away. But, the primary problem is that you're not trading even one player or two players who aren't as good as Allen, you're trading more than that.
The additional cap space to sign Allen vs. 2 low first rounders would be another player or even two. Is Allen really worth three other guys? Especially young guys who might wind up being worth as much as him? I would say no.
BTW, you were pointing out that Hackett and Johnson are still asking for more than they're worth, and I agree. But my primary point is that, while they still may be asking for too much, they probably aren't asking for as much as they were two weeks ago. And I bet that the amount they're asking keeps going down with every passing day.