Forget about the historical achievement bonuses for now, and take a look at the year-by-year breakdown of the deal (including signing bonus)
This deal makes sense. A-Rod will be paid like the best player in the game for the next 7 years or so. If all goes according to plan, he'll also play like the best player in baseball over that period of time. His salary tapers off dramatically at the end of the contract, when his production will most-likely taper off.
The numbers may seem staggering, but look at it this way, the Rangers signed A-Rod to a 10-year, $252M deal in 2000. Without the historical achievement bonuses, this contract represents a 9% increase over the deal Texas gave him. As a point of comparison, a gallon of gas cost $1.51 in 2000, today the national average is twice that.
Here's what A-Rod said about opting out:
He said he and Boras agreed the slugger would opt out of his deal, but Rodriguez thought the move would be revealed after the World Series.
"I made mistakes. I've got to look in the mirror. If I had to do it again, I would've called Hank from Day 1 and negotiated myself," Rodriguez said.