This is what BA had to say on the subject recently.
Appel and Oklahoma righthander Jonathan Gray have established themselves as the top two prospects in the 2012 draft. While Appel has less leverage than he did a year ago, his talent still gives him plenty of negotiating power. He should get much more than the $3.8 million he turned down from the Pirates after sliding to the No. 8 overall pick a year ago.
Appel’s situation isn’t really analogous to Correa’s. If Correa hadn’t gone No. 1 overall to the Astros, where the assigned pick value was $7.2 million, the perception was that he would have lasted until the Cubs at No. 6 ($3.25 million) or the Padres at No. 7 ($3 million). So it made sense for him to take a discount that still paid him more than he would have gotten otherwise, though I’ve since learned that had Houston taken Byron Buxton, the Twins might have chosen Correa at No. 2.
The best-case scenario for the Astros is a repeat of 2012, when they land a player worth of the No. 1 selection but save enough money to do a lot of damage in later rounds. This year, Houston’s assigned pick value is $7,790,400. If the Astros pass on Appel, he very well could be at the top of the Cubs’ draft board at No. 2, where the value is $6,708,400. The Rockies (No. 3, $5,626,400) and Twins (No. 4, $4,544,400) probably would be thrilled to have Appel and willing to move money around to add to their pick value. So he should clear well north of $4.8 million.
There are two good reasons that a team won’t take a hard line with Appel. First, he’s advised by the Boras Corp. Clubs legitimately fear that draftees, even college seniors, will walk away from a deal that Scott Boras doesn’t like.
Second, Appel could take a hard line with a club. Let’s say the Astros choose him and make him a take-it-or-leave-it $5 million offer. I agree, it would be silly to turn that much money down to re-enter the 2014 draft.
But Appel could decide not to sign until right before the July 13 signing deadline of 5 p.m. ET. Houston couldn’t risk spending the $2,790,400 they saved on his bonus to land other players until he was in the fold. (Update: Actually, as @jeremynygaard reminded me on Twitter, the signing deadline doesn’t apply to Appel, making it even easier for him to hold a team’s bonus pool hostage.) If the Astros spent the savings before Appel signed, they’d risk losing their next two first-round selections, including the possible No. 1 overall choice in 2014.
Ultimately, clubs give up leverage to draftees because they want to sign the best players available. If Houston rates Appel and Gray evenly, then it makes sense to select the guy who will accept a more team-friendly deal. But if the Astros like Appel more than any other 2013 draft prospect, I think they’ll take him and give him a bonus of $6 million or more.