Your work is already protected under copyright law. It affixes automatically upon completion. Registering the work with the US copyright office, however, is very important because you cannot bring an infringement action to a court unless the work is registered and only those works registered before the infringement occurs will qualify for court awarded statutory damages, often the only damages that can be shown.
The registration fees are $35. If you cannot afford $35, and I am saying this to be pragmatic not to be a wise-guy, you probably have much more important things to take care of than shopping a script.
DISCLAIMER: this is not intended to be specific legal advice and should not be relied upon as such. No attorney-client relationship is formed with the law firm of Natoli-Lapin, LLC on the basis of this posting.
Registering a copyright provides evidence to a court that you are the creator of the work. Copyright law already protects it, but proving when you came up with the idea and wrote it down can be a problem. Registering your work with the copyright office goes a long way to eliminating that problem. If you are in the business of screen writing, the copyright fee of $35 should be viewed as a cost of doing that business.
Any legal issue can be complex and the answer can vary by jurisdiction. Before taking any action, you should consult with an attorney who is licensed to practice in your area. The information above should serve as starting point for you to contact an attorney to review all of the facts and give you a legal advice based having the complete picture. The hiring of an attorney is an important decision that should not be based solely upon advertisements. The information provided by this website is not intended, nor should it be construed, to be legal advice, nor is it intended to not establish an attorney/client relationship.