(In some fields it's common to have strict pay grades with clearly defined criteria while in others it's much more flexible. A salary advisor would probably be field-specific, and it would make sense for them to pick a field where amounts are on the variable end.)

Incentives are most aligned if they're paid a fraction of the additional compensation they help you negotiate. Which isn't possible, but an agreement up-front like "X% of every dollar over $Y", where $Y is a number you think is what you could get on your own, might be good.

(I don't see anything like this out there, but it's hard to say for sure because the search results are so full of scummy seo minimal-content lots-of-job-ads results. You can do this somewhat self-service with places like glassdoor and there are compensation specialists, doing this work on the other side in HR, but the potential compensation range in some fields is large enough that it would probably be worth hiring an expert.)

Update 2017-05-18: Someone made something like this.

[1] Maybe it would make sense for them to offer to pay small amounts for salary details, bonus amounts, etc. And they could collect current ones from anyone asking for advice.