SEO can take many forms. What services do you provide, specifically? Some SEO companies only do link building; others completely rebuild the client’s website and perform a multitude of other supplementary tasks, like monitoring the client’s keyword performance on a regular basis, managing client communications, doing Adwords and other PPC campaigning, and optimizing content. Once you know what kind of service you provide, you can compare yourself with others. This also affects the kind of guarantee you can offer to clients.

You need to do some research on your competitors. Here is how I would do it.

Create an account on eLance and submit a request for proposals to SEO your website. Let them know in your request that you will need a specific proposal with costs.

This should give you more than enough competitive information to work from.