This article is from the misc.writing Writing FAQ, by Wendy Chatley Green firstname.lastname@example.org with numerous contributions by others.
Agents should not charge authors up-front fees for
copying, telephone calls, et cetera; this money should come
out of the agent's percentage of the gross. The standard
agent's fee for fiction seems to have risen to 15 per cent.
Agents' fees for screenplays are reported to have remained
at 10 per cent.