If you discount everything back to today, the net present value of the Senate seat would be ~$6.2 million.
Here are the calculations. One way to dispute the numbers is to adjust for the opportunity cost of the talented labor of the would-be Senator. Still, with finance falling apart, the job market slowing down, and the option of receiving bids from wealthy but non-talented people, I remain surprised that 500K would be viewed as a going offer. The associated fame and power is fun. Clearly the seat was worth more than that to its previous holder, no?