There's only 2 problems with that scope: 1) it's old, and 2) it's a Simmons.
No, seriously, I may not be a Simmons fan, but I guess it might have some kind of "value" to it. On the other hand, just because you bought it for $15 doesn't necessarily mean that you got a great deal. Why? Well, I have seen top-of-the-line Simmons scopes fail (and I mean more than just a few). As such, even if you got it for free means that it still is might be worthless because it doesn't work. From a pure "collector's" approach, I honestly don't know what it's worth.
Zachary