Ignoring sentimentality, if Zimmerman comes back for under $10 million he is a good risk at first base. Good defensive player and I wouldn't be surprised if he has an OPS around .800 next year. Any other first baseman with any hitting ability who isn't a net negative defensively and on the bases would cost a lot more than Zimm.
I think we should try to bring back Zimm too. He'll probably give us at least 50 starts, good defense and good hitting, and he could be a good late inning defensive replacement at times possibly. However, since we can only count on him being available part-time, I don't think we can afford to pay him too much. Maybe something like $5 million maximum.