How much would it be?
You need to obtain reseller permits in states where your company has nexus (economic or physical). Usually nexus is created in the state(s) of registration, and in (certain, such as California) states where there is significant economic presence without physical presence.
Most states don't have state filing fee for reseller permits, while a few do. To know exactly what it would cost you please tell me more about your business, and we will analyze it nexus-wise.