The current calculation is bitcoins/sqrt(time)
The spammer is still just a single person, and if no one is sending coins to lets say his "viagra" post, then even if he has an advantage, it only takes a few people to like something else to outgun him. How many people vote up something on the home page of reddit. If they all put in for that, it would be very uneconomical to compete with.
Keep in mind, I don't really have a huge amount of faith that the system will work out. It is an experiment in micro-transactions and psychology.
Time will be the best judge.