It depends on what you want to do with it. Judging from the age, the fact that it's a student horn, and the look of the brass, I would imagine it has a relatively thin tone to it. So if you just wanted something to toot on now and then, $100 (as of now) is a nice price.
That's assuming of course, that it doesn't need much work done to it. He did say it'd been sitting in a closet for a long time, so I would think at the very least you'd need to put another $50 to $100 into it to fix any regulation problems that are almost certain to be there. And it's also assuming you don't need a case. A case would run you another $75 to $100.