The 49ers typically have a 40-percent roster turnover from one year to the next. The goal is to keep the nucleus intact while cycling through players to find a combination of young players on their first contracts and veteran role players to go along with the nucleus. We turn back the clock three years to a statement Trent Baalke made shortly after becoming 49ers general manager: ”I think it’s always important to keep good football players and we’re doing to do everything we can to keep as many of them as we can. But the fact remains, it’s a business. And you can’t keep them all, you can’t pay them all, you can’t make everybody happy.” The question is always asked: “Who is the 49ers’ top priority to re-sign?” The answer is never simple because the 49ers have assigned a certain value for every player. When the player’s asking price exceeds the amount the 49ers have set aside, the 49ers move on rather than take cap space away from others to satisfy someone else. Anquan Boldin and Donte Whitner are two starters the 49ers want back. But if Boldin is looking for a significant raise on his 2013 salary of $6 million or Whitner seeks Dashon Goldson money, it seems unlikely either will be back. (At least until they hit the open market and see if there are other teams willing to step forward to meet their demands.) All indications are that Boldin will be easier to re-sign before free agents can begin negotiating with other clubs on March 8. More important for the long-term future of the 49ers, there are four significant 49ers players who are entering the final years of their rookie contracts: Quarterback Colin Kaepernick, wide receiver Michael Crabtree, guard Mike Iupati and outside linebacker Aldon Smith.