Post up a watermark sample of them to determine if anyone is interested.
Price really depends on quality and number of sprites. One sprite will never really sell for more than $10, and $10 if it's really good.
"packs" are what sells, especially if they are customizable packs I Bought a spaceship pack once for like $20 and it had a bunch of different parts you could use to made different ships, was pretty cool and made them original.