The job of an experienced recruiter is to help educate the candidate by speaking to actual marketplace knowledge of similar searches that you have done in that geographic region. In addition, the client may also offer a strong bonus and benefits package that may offset a base salary that may not meet expectations. The recruiter may also point out that the client offers a stronger career path than the current company does. The job of an experienced recruiter is to get the candidate to focus on the entire package that the client is offering.
At the same time that the recruiter is working to educate the candidate, a similar process may have to be done with the client. In many cases, the company is very concerned about internal equity and base their understanding of what to offer a candidate based on what employees are making in a similar role. However, if they have not hired for this particular role in awhile, the recruiter will need to educate them on the going market rate and what other clients are paying for similar roles. In the case of a client that has never hired a certain position in the past, the client needs to rely of the recruiter to guide them as to what the market is paying.
In the end, if both the candidate and client want the situation to work, an experienced recruiter can be the bridge to make the negotiation of the job offer successful for both parties.