I don't think Posey will get $10 million, the way he's rumored to be asking for. But even if G's offer him $2 million, that's a lot of money for a 20 year old. I don't think he would go back to school, risk injury and lose the chance for a nice pay day.
Gillispie wasn't picked as high as he expected, but he'll still get a nice signing bonus at the spot he got picked.
The 3rd and 4th rounders are more unknown. They can go back to school, and if they do well, they might be 1st round picks next year and get paid a lot more.