I think the high school guys being conjectured would be at least at slot and in some cases maybe a bit more. The extra pick gives them some ability to move money forward and take a low-ball senior sign at #70. I imagine there are multiple scenarios in play depending on who is there at #24. They likely have everyone's "number" as to what it would take to get them to sign.
Didn't see it mentioned, but Longenhagen at Fangraphs has us taking Kent State LHP Joe Whitman at #24. He's #37 in the MLB rankings (as if that really means anything). Probably underslot.
EDIT: I see CJ9 checked out Fangraphs right about the time I did.