Through a friend of a friend, I stumbled into an estate that had about a half-million dollars worth of Argentinian art and I ended up getting to handle the estate. Turned out to be quite an experience. The heir to the estate was not close to her brother and had no idea what he had, so she was quite pleased (to say the least).
Anyway, if you have something big-time auction houses would definitely want, my suggestion would be to contact a couple and play them off of each other. Be upfront that you're shopping around for the best deal and they'll be competitive with their consignment fee. I dealt with Christie's and Sotheby's, let them know I was contacting the other, and they each came back with a proposal. One was at about 5%, the other at 6.5%.
I'd be surprised if they'd be willing to waive fees entirely -- unless you have something out of this world. They deal with high-end stuff all the time so it would have to be something pretty special, I would suspect.
As for taxes, that sucked.
For handling the estate, I got 25 percent, so it was a once-in-a-lifetime kind of thing for me. Like a year's pay in a day. But I ended up writing a ridiculous check to the government for taxes. That part of it was very depressing. But hard to complain. It was a great experience, and pretty lucrative. Hope that helps. If you have any questions about specifics, feel free to send me a message.
Rob