Tack on an extra 20% for each year and that's more like what DJ is going to make. The CBA allows for anywhere from 80% to 120% of the rookie scale contract amount. After talking with the Director of Scouting for the Spurs a while back, he told me its very rare if the player doesn't get 120%.
I didn't realize the last CBA gave team options for the last 2 years of rookie contracts. 1st round draft picks could conceivably get cut after 2 years with a team now.