The answer is dependent on what services you will provide for your client. I have found that people do not always understand seo and therefore question you when you charge a fee for your services. People pay for a number of different services in business, why should seo be any different?
What you need to do is give a detailed explanation of all the services you provide in a document with the number of hours it will take to complete each one and your hourly rate. As long as it is clear, you should not have any disputes. You should refer back to your proposal in your costings so that the client knows exactly what services they are getting for their money.
Do not doubt yourself or try and amend the hours. If you have worked in seo for a number of years, then you know how long it takes (realistically) to complete a piece of work. Make sure that what you quote is how long it takes to complete the task and keep the number of hours consistent for that client. Do not make up 5 hours for a piece of work and then change it the next time.
Within an seo proposal, you should also incorporate ppc and social media. 2011 is the year for integration. It is important to rank high in natural search for those terms that you may be spending too much money on with paid search. The lines between social and seo will become more blurred this year and your social media presence will have an effect on your rankings.
So in conclusion, when you are putting together a proposal, make sure you are clear on the tasks you will complete for the project and do not be afraid to charge the correct amount for it.