There's two major considerations when it comes to MLB teams drafting. One is their management. How is the organization run? Do they give players fair chances in making it to the MLB level? Do they make the right decisions about moving players up & down or make the right trades based on logic?
The other consideration is depth or players in the minor league system. Does the MLB teams minors have enough depth at the position they draft? It has nothing to do with the MLB club level depth at the time of the draft. It's whether the players drafted have a better chance of advancing through the minors because of depth issues or players just not good enough already as starters.
Those are the two main things to consider in drafting a player. To me, I said Senzel would fit better at #6 when in reality he would be better off with going #9 to Detroit which is a much better run organization than the A's. Then, you bring up the money difference though. Yes, big drop-off in pay the further you fall down the draft board.
If Senzel was drafted by the A's at #6 he would be looking at around $4.5 million probably. The Tigers at #9 like you said falls off to ~$3-$3.5 million range. The Reds take him at #2 then yes he's looking at major bucks. $7 million+ wouldn't be out of the question.
Of these 3 teams though as far as being better off he would fit better with the Tigers because of their management & minors. He would be losing an awful lot of money though & I'm sure he doesn't want that.
I would love to see my Braves draft him at #3 overall. That would still be really good money. However, the Braves management is in shambles. They have the top minor league system in MLB in terms of players.
May take him longer to get to the bigs being drafted by the Braves but they way we are I woudn't want him to go there because of the situation. I want him to go to a team that gives him the best overall chance to make it to the MLB club.