Depending how the third party contracts are set up, it's possible that Sony would have to pay them the normal amount per download, regardless of what the price is. After all, it's unreasonable to expect the third party to be in charge of Sony's network security.
If there's a glitch in the Walmart computer that sells all the Wii games for a penny, do you think Walmart gets to then pay less to their distributors for the games?
In the end, it could be a huge boom to the third parties, if their games get a ton of downloads, there's a ton of happy customers and Sony still has to pay them full price. :D