this article at business insider places the value of a winshare at 1.47 million dollars, and last year batum accrued 4.8 winshares in 59 games. that extrapolated over 82 games would be 6.67 winshares (oddly enough, batum accrued 6.6 winshares in 2010/11 in 80 games)
soooo..... it stands to reason then that batum has already been worth roughly 10 million dollars production wise over each of the last two seasons (6.6 winshares *1.47 million per winshare = 9.7 million)...then on top of that you are also likely paying a premium for his potential due to his age, so 11 or 12 million a year seems like a reasonable offer sabermetrically speaking
if he gets that offer remains to be seen