how do you price investigations?

If a person comes off the street (actually found us over the internet) and asks you to take over an ongoing investigation where other accountants were involved before and the alleged tax lost with penalties could be over a hundred of thousands, (because according to him his normal accountant doesn't have the expertise), would you take the case? And if yes, how would you price it? E.g. a fixed price or a fixed price with option to revise should the work exceeds quote (seems reasonable to me), any payments in advance and so on? And, most importantly, what are the current market rates for this sort of work? This case could go to the Tax tribunal eventually. Thanks.