most - not all - of the consultants people i know have had dealings with cost around $2500 and up and gave little or no results. when i do consulting, i either charge an hourly fee with each part of the work being approved in writing by the client or else i quote by the project after finding out what needs to be done.

if you go with a consultant, get some references. there are people out there who charge a flat fee and imo that isn't possible since every site, niche, situation and webmaster is different.