Signing bonuses are determined by many, many factors...
If the Nats paid $500k for last year's #2 overall, that doesn't mean they will pay more or less this year for the #55 overall. For example, the #2 last year could have had NO ALTERNATIVE other than baseball(no FB offers, grades not good enough for D-1, so his only BB option would be JUCO, etc.) so they low-balled the bonus & he gladly accepted his $500k. Also, if he was in college, that could affect his bonus offer: Juniors drafted out of college tend to get more $$ than anyone because they are proven, yet can go back to college for 1 more year.
All this being said, the NATS are going to have to ante up if they want DH. He has options...very good options in terms of bonus $. I understand $500k is a lot of money, but if he feels like its not enough in the long run, or that he could get more $ from a different club, then he will probably wait, come to bama & take his chances.
IMO, he won't get the amount he is looking for, but he will go anyway. Dreams are dreams & injuries happen every minute of every day. MLB is where the long-term career is & $500k is a lot of money for that kind of shot.