How good of a sales man are you? How well can you use the tools of the seo trade to research and develop a backlink plan for the client? If you are on top of things, factor in all the software prices and how long they are going to need you to use them. Also, outsource as much as possible and include that in costs. Your average should be around $2,000 a month if you are on your game, and less than $0.50 if you suck.