Everything?Everything is negotiable.
But no one is obligated to accept a lower figure.
I could see being offended if a guy was insulting in his offer, like your rate is 300 an hour and he offers you 40.
Perhaps it would make more sense to stick to the rate, but get more scope of services for that rate.
Do you negotiate with the ticket seller at Wonderland?
With the manager at the grocery store?
With the restaurant owner about the price of your meal?