From Matt Maiocco's blog:

I have correct salary information on linebacker Tully Banta-Cain, who signed Tuesday with the 49ers. Both reports yesterday are partially correct. Here's the deal: Banta-Cain gets $2.9 million to sign, with a $500,000 roster bonus and a $100,000 workout bonus this year. He'll make around $4 million in the first year. He is scheduled to earn around $9 million in the three-year contract, but if he reaches some easily attainable sack numbers, the total package jumps to $12.25 million.

Matt's blog, Instant 49ers, can be found here:
http://49ers.pressdemocrat.com