I used to work for a recruitment agency, and yes they do charge whatever % they want to secure the deal, and your pay rate comes out of what the agency makes, so if you ask for a higher rate they will lose money out of that deal they made with the client... iykwim?
When i did temp work I would just say the rate i wanted and i wouldn't go lower, so maybe you should just ask for what you think you are worth, or how much that particular job is worth and ask your agent about it, she/he will then have to go back to the client and renegotiate the deal price. If the agent comes back with a firm no i'd keep pushing, and explain all the extra responsibility etc.
You could look on some employment sites like Seek and see how much other employers are paying for similar roles? At least that would back up your request for a higher rate etc...
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