Yeah you can buy a cheap new pump for under $200 and only $60-$50 more than a single shot, but its a ***!
I have held them and wouldn't take them across the street to hunt let alone have them defend myself with. Now I did pick up a few Mossy 500's cheap. Couple for just $150 each. I knew they just needed a good cleaning. The seller knew they were useless due to the old camo pattern. They now sit in my closet and beside my bed ready.
The only thing I have seen fail on a 870 is the shell interuptor thingy. So I would have on of those in hand. The 500 is just as cheap to have an extra for spare parts.