Wait, have you been paid yet for those three projects? If not, then obviously, you should even consider giving her a dime.
If you did get paid, offer her 10%; which is a generally fair amount. If that's only $10 per project, so be it. Tell her to find you better paying projects. You should only pay commission to the agent, not any upfront fees. Get your own head shots too, agents tend to work with photographers and have been known to inflate the price.