I'm sorry if I seemed to imply that attorneys were somehow dishonorable, and would deliberately stiff you for a fee. Most of the attorneys I've met are very scrupulous; they won't willingly engage in fraud or chicanery, for the same reasons that we won't. They are, for the most part, principled. During a trial, they're extremely focused. After the trial, they're tightly focused on the next one. Trivial details like paying the light bill escape them, unless their secretaries take care of it. That's why you get your fee up front--because they're busy, and what is behind them is not important.
So get your fee up front, avoid hard feelings, and perhaps make a friend and ally in the process.
What to charge? Well, how good are you? Do you have an extensive CV with several teaching posts, book authorships, magazine articles, and government postings to your credit? Charge a small fortune, because you're worth it. Don't have much of that? Tell the attorney you have no experience as an expert witness, but that you'd like to learn. What would you charge the attorney to teach him appraisal techniques? What do you think he should charge to teach you courtroom techniques? Consider doing a couple of freebies to get the experience in the witness chair, if that's what it takes. (Free court appearance, not free appraisals.)
To give you an idea of rates, I quote $150/hr portal-to-portal, plus expenses, for limited experience as a witness (negotiable). (We did our first one at $50/hr flat for a divorce.) I know a highly experienced MAI who gets $1,500/day or portion, portal to portal, plus expenses, payable in advance. He also commands a retainer over and above that fee to compensate him when he's likely to have to turn down other business due to conflicts of interest. He teaches appraisal courses, either to appraisers or to juries. That's part of the job of the expert witness, to be able to show the jury what you did, and why it's the right way to do it; in short, you teach a short course in the appraisal approaches. Know'em cold, though, because then comes cross examination. That's where you prove your worth.