What do PR agencies typically charge for social media management for an event - pre, during and post social media management?

I am a freelancer and need to understand what PR agencies would typically charge for event social media and price accordingly as a freelancer. 3 day event. Facebook, Twitter, Instagram, SnapChat networks will be used, plan to charge per phase (pre event social media, during event social media, post-event social media, includes taking pictures, video, responding to community interactions/comments/questions), may offer optional support - i.e. web and email copywriting, social media metrics reporting post-event.