I would look at used rifle prices in a similar condition to the one he's offering you to get a base price of what it would be going for in a similar condition with all the parts. Figure the cost of the parts and subtract that from the value of the gun if it were whole and offer him that. The last thing you want to do is get it for "a good deal" and then spend so much in parts that you could have just bought a) a good, used rifle in working condition or b) a brand new rifle. The Mini's aren't that expensive relatively speaking. You can get 'em at Walmart all day long for $700 OTD. I'd say $500 with all parts and everything is about the most I would spend.
Three old hunters were walking through the woods one day. The first looks over at the others and says, "Windy isn't it?" The second says, "I was sure it was Thursday." The third says, "Me too! The heck with hunting, let's go get a beer!"