Avery Bradley's injury is a legitimate reason to want to bring back Ray Allen for one more year. Bradley's recovery timetable is four months, which brings him up to the start of training camp. If his recovery is slower than anticipated, the Celtics need someone who can step in and start.
If you bring back RA on a one-year deal, then you're also bringing back KG and you're probably not clearing cap space to sign a free agent, so it might be a good idea to overpay a bit with a wink-wink deal that he will give the Celtics a bit of a discount if he is worth bringing back beyond that. So, a respectable amount to offer might be something in the range of salaries for players who finished in the top ten in voting for Sixth Man of the Year. Beyond that, he might give the equivalent production of a taller Eddie House who isn't a good defender but isn't as much of a liability against taller SGs and he might be worth around what House was getting paid as a Celtic.
I think there are places he could be a 15-20ppg guy. I'm imagining how he would play if he ended up starting in Brooklyn next to Dwight Howard and Deron Williams.