Bill by the time (average) to create the first document and then multiply it by quantity. Build in some buffer so not to under price the project should one or some of the the project segments (aka posters) take longer to design than expected. A discount for quantity is warranted on a per client basis (one-off client vs long term / on-going business relationship, etc.)
Transferring full rights, which essentially means loss of billable time for changes over the life of the content, should be factored in to the contract price. Offset the discount that is offered, if any, rather than having a "rights transfer fee" as that can look petty. Increase the contract by a percentage of potential loss in billable time. Again, this is per client basis.
Update the contract for production to reflect a written agreement as to what assets are transferable and what is not. One particular asset requirement might be a "Design By" label giving recognition of your creation not be removed; alternatively, you might not want your name on it once changes have been made thus a stipulation that the Design By label be removed upon change. Appraise the project by content and ensure you do not loose rights to a potential asset that is otherwise utilized in past or future projects.