IME, owners of higher priced properties are the least likely to negotiate the commission rate. Very few agents are capable of selling these properties at a high value and you need a selling agent with lots of the right connections. Just listing on MLS isn't going to do it, there's a lot of cold calling.
Also, if it is luxury, you will want a listing agent who will be there at the showings and vet the finances potential buyers prior to arranging a showing. Theft is a significant problem with luxury properties. It takes a good agent to make this process feel exclusive and value-add for the buyer rather than a screening process for the seller.
You don't want to underpay the agent who brings your buyer or you may find yourself without that buyer.