If you could find an attorney willing to take your matter on a contingency at this stage, my guess would be the percentage would be quite high - above 40%. The reason is you are asking someone to agree to not only handle an appeal where there is, at best, a 20% chance of success on a good day, but then also handle the case which underlies it if the appeal results in a new trial. The biggest problem will be that most contingency lawyers will be unwilling to take over at this stage, given the amount in controversy and the high probability of loss.
If, on the other hand, you are simply asking an attorney to give some advice but not take over the case, what that attorney would charge is negotiable, again if you can find someone to do it.
Remember that contingency lawyers look at new engagements with a keen business eye. They weigh potential fee verses many factors including the risk of not getting paid, the number of hours it will take, the exposure to costs, the exposure to their reputation, the client's demeanor and nature, and many others. It is a business decision, not a decision of the heart. It has to be because if attorneys made decisions of the heart, they would go out of business very quickly and be unavailable to help anyone.
If you wish to get services from someone where they do all the work with a substantial risk of loss of their time, you will have to offer a fair percentage of the potential recovery to attract that person.
I wish you the best of luck.
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I have already explained to you that we (appeals attorneys) do NOT work on a contingency basis.
In some states, there are ethics rules prohibiting it. So, if there is anyone of us who will act as consultant, he/she will expect to be paid either in advance or by the hour. (My time is worth a minimum of $150/hr.)
Cal Bar No. 104800
Wis. Bar No. 1020123
Member: U.S. Supreme Court Bar
Actually, this is a contingency (winning appeal) based on a contingency (winning at retrial) based on a contingency (a solvent defendant to collect from). There is no standard fair amount. If any lawyer offers to help you should seriously consider.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.