Well, to make sure the client feels safe you need to be transparent. Let us say, for example, if you are providing SEO services, you may provide your client with the list of services inclusions you have and the breakdown of services payment it may cost. As well as a regular report on your progress. Apart from this, reassuring constant updates to your client is a great factor.
Some SEO and managed website services company choose not to list the full quote breakdown in order to make it appear lower, yet there are some unexpected costs which often results in an abandonment of the service.