The treads had the same orange-ish looking stain on them that the floor originally had so that had to be sanded off. The upper stairs wasn't nearly as gnarly as the lower one and was pretty easy to sand.
I stained the treads with some natural and used two coats of polycrylic after that. They looked so much better for so little work I decided to tackle the stairs going to the basement which are in far worse condition.
Oak treads are about $25 apiece now so all twelve would've been $300 bucks.
I broke several of the treads prying them up and had to glue them back together which wasn't really a big deal. I had about a five foot long two by four and I baited it up with some short pieces of drop and they pried out lickety split.
The space behind the upper stair was filthy. I think they swept the first floor dirt under there before they sealed it up. It was like a time capsule with Coke bottles and soda cans.