It wouldn't surprise me if he originally said he was going to opt in simply because he wants to come back and believes it will be the best decision, but now that they are in the guts of free agency he's working through the process with the team and his agent.
Green appreciates San Antonio for the original chance, the life he has, and how much he enjoys the franchise. A lot of guys are team members on the court, but everything I've seen from Danny indicates that he believes that being a team is about a lot more than showing up for work.
Undoubtedly whatever he does, he will do having discussed it with the front office first and the front office will follow through on what they tell him. I don't know that he would take a huge hometown discount, but if the money is anywhere close he stays. Certainly if someone comes in and offers him double what the Spurs can they'll tell him to take it, but Green is worth more to the Spurs than he is to most teams so I doubt that will happen.
He's 31 now. Say he re-signs for three years. I think he has a much better chance of getting another solid three year deal when he's 34 if he signs with the Spurs compared to anyone else, so I'm sure that weighs into his decision-making process even on a pure dollars-and-cents calculation.