It depends-- do you need a lawyer to put a prenup together, or are you just looking for a lawyer to witness the signing? I'm not sure why you would need a lawyer to be a witness in any particular situation.
Disclaimer: The foregoing is not legal advice and we have no attorney-client relationship. Every case and matter is different, and I made a number of assumptions in generally discussing your situation.
You don't need a lawyer to simply witness a prenup. Anyone can witness.
You should have an attorney to draft, review, and negotiate a prenup. The fees depend on the complexity of the document. If you need additional information, please contact me.