There is a huge variation in the cost of mediation, depending upon the nature of the dispute and the way the dispute got to mediation.
Disputes between neighbors may be mediated by a neighborhood dispute resolution organization that provides free services. For instance, in San Francisco, there is an organization called "Community Boards" which has been training mediators and conducting free mediations between disputing neighbors for twenty years.
When a court requires litigating parties to mediate, it usually has a list of volunteer mediators who will work for free or a nominal fee.
In other cases, where the parties want a professional mediator, they can expect to pay from $1,000 to $2,000 each (per day) for a mediator. The costs are usually split equally between the parties.