The only coins that affect prices are the ones being actively traded (the effective "supply"). Coins that are not in circulation (either in "lost" wallets or just being held) do not really have a significant effect.

Edit: The coins not in circulation don't affect supply (and therefore price) until they are brought back in. Although it's possible that the risk of these coins coming back can decrease prices slightly, I doubt traders are thinking about that fact right now, so its effects are probably negligible.