Fair market value for an old 870 isn't going to include sentimental value. If the gun is in really good shape externally and internally (that is, almost like new) then $300 or so is a ballpark figure in this area (for a 2.75" chambered non-vent ribbed gun). The more worn it is outside, or the more rusted if it has been neglected, the less it's worth. And value tends to go down to about $100- 150 for pedestrian guns with finish damage but fully functional.
There's no need not to be fair to yourself while you are doing your friend's family a favor. Pay him what the gun is worth as a fair price on the local market. If it is to be an interest free loan over a period of years, then you do not want to cheat yourself on the collateral for the loan as well as giving away the interest.
I applaud you for your thoughts and intent, and i hope you find someone in the family with the good sense to treasure an heirloom that is handed down... literally a gift from
across the years.
Note that is is no accident I use that particular phrase. Some of us here seem to be genuinely entranced by certain writers who share our own faults and foibles where fur, feathers and the smell of gunsmoke and wet dogs are concerned. One of my own favorites, literally since I was a boy, is a gentleman named Corey Ford. Many years ago he penned an article for
Field and Stream, which in those days hired real gunwriters rather than fops, elitists, treehuggers or Fudds.
You can still see this article online, as it is a classic. Please go to
http://www.fieldandstream.com/fieldstream/hunting/article/0,13199,394327,00.html and give this one a read. You might want to share it with your friend as well. I hope you enjoy it!
lpl/nc