Jerry Mathers said in an interview (On a bonus section of the "Family Affair" DVDs, where they interviewed child stars), that after the last season, he was offered a choice to do another TV sitcom for a lot of money.
He told his parents he would rather just go to high school and play football. His parents told him to be sure as he may never get another chance, and he said he was and that was that.
He also said, while he never made a great deal of money, he had enough to buy a car during high school, another car for college, the show money, paid for his entire college education and he even had enough left over to put a down payment on his first house.