I have registered a lot of my art on a single application for one fee because I grouped it as a collection and gave it a collection title.
See these links
http://www.copyright.gov/eco/help-type.html
An excerpt:
What Can Be Included on a Single Application
For published works, all works published in a single unit of publication and owned by the same copyright claimant may be the subject of a single registration.
Collection of Unpublished Works: Registration of Multiple Unpublished Works as a Single Claim
To register a number of unpublished works with a single application and fee, they must be grouped as a collection under a Collection Title and must meet the following requirements:
• At least one author must have contributed to every work.
• The ownership of every work in the collection must be the same. This means that the authors of each work must be the same or they must have transferred copyright to each other or to a common claimant. If there has been a transfer of ownership, the application must include a transfer statement on the claimant screen. If ownership of all the works to be registered is not the same, one or more separate applications are needed to register all the works. Each application requires a separate fee.
http://www.copyright.gov/circs/circ44.pdf
An excerpt:
A single registration can be made for cartoons published as a unit (for example, a comic book), provided that the copyright claimant is the same for all elements in the unit. Two or more unpublished cartoons or comic strips can be considered for registration as a unit on a single application when submitted with a nonrefundable filing fee and a nonreturnable deposit of the work if the following four conditions are met:A single registration can be made for cartoons published as a unit (for example, a comic book), provided that the copyright claimant is the same for all elements in the unit. Two or more unpublished cartoons or comic strips can be considered for registration as a unit on a single application when submitted with a nonrefundable filing fee and a nonreturnable deposit of the work if the following four conditions are met…