Placeholder assignments take their charge out rate from the role rate of the project they are on. They are the same as normal assignments.
For example, if you add a Developer to Project A and the Developer Project Rate is $150, then the placeholder will have a rate of $150 too.
Placeholders have a cost to a project but not to the business as a whole. The cost comes from the "Default role cost" which can be changed by editing a role.
When you are looking at a single project or the project report, they will have a cost.
When you are viewing overall profits and costs (e.g the planner graphs) they do not have a cost (because Runn does not know if you will hire someone external for the assignments or use someone internally).