How much should you charge for what you have to say? When you're negotiating the price of something whose value is not set, bear on mind the following rule of thumb: the first one who mentions an actual $$ amount loses. Every time. So, answer, "how much do you charge?" with, "Before we talk money let's go over in a little more detail how I can help you?" In other words, establish your value in the mind of your potential client.
Another rule of thumb: if this gig isn't a fit, recommend someone else who is a fit and walk away- leave goodwill in your wake. You may fondly hope that you're God's very own answer to the speaking industry, but you're delusional if you think you and every speaking opportunity in the world represent a match made in heaven. Hey, sometimes the answer to a yes-no question is "no!"