Is there a fee to use Suricata?

No, there is no fee to use Suricata. You can use it personally and in commericial environments without cost per the terms of the GNU General Public License (version 2).

If you are redistributing Suricata (for example, using in a product and not just using internally) then you may have a use case that does not fall under the GPL and should contact the OISF. Please see GPL: Frequently Asked Questions - Suricata for more information.