"I always do a "what's it worth to me" evaluation rather than a "what did I pay for it" evaluation."
I completely agree. The cost of the gun has absolutely nothing to do with buying accessories, or paying a gunsmith to make the gun into something that pleases you. Nothing personal, but I can't see shooting a gun I am not satisfied with only because it cost me $100 and I am not willing to spend any more money on it based on the price I paid for it. Would it change anything if you had paid $500 for it ? You can buy a new 870 and the stock still will be too long. My primary hunting rifle was purchased new and the scope cost more than the rifle. My primary AR15 was bought new and I paid more than twice that much for the accessories including optics for it. So what ? If I didn't spend that money, I wouldn't have what I wanted. The way I look at it, you are ahead of the game. As I mentioned, you could pay $300 for a new 870 and have the stock shortened or you can pay $100 for an 870 and have the stock shortened. Either way you get a gun that fits you, but with the second option you did it with for $200 less.
I hope that wasn't offensive, it wasn't intended to be.