I'm not sure the 512 shares out of ~100,000,000 shares in my example is worth anything. My example is worse than that as it is 512 shares in 3 shifts (~300,000,000 shares).
Unless the price of Bitcoin (currently ~$450) goes back up, CPU mining is just a little above break even (unless you get free power). I just turned my Primecoin miner off as I think it is a loss for me after A/C costs.
Edit: 1 Share at 512 diff is 512 shares (I think)
This is similar to what I'm seeing. Even with 100 cpu miners, I maybe get in 1 share every 2 blocks and all the old work is rejected as the other CPUs take too much time to finish their share and then when submitting, it's too old. At least that's how I understand it works.
So it seems in the end it is a mix of the high difficulty + such a short block time on certain coins. I imagine that if Wafflepool starts mining on an altcoin with a long block time, my rejects would go away.