Third time is the charm?

How much does it really cost to submit a piece of prior art to the patent office for consideration? Since they are trying to encourage people to submit prior art on patent applications which have not yet been granted, they ought, for completeness, to have a similar system for submitting it after the fact. Perhaps charging a fee for it which was high enough to stop them being inundated by silly submissions (and perhaps even high enough to make it financially interesting for the patent office), but low enough not to discourage any serious submitter.