Serious Influencers work on their Influencer business and their accounts full time. That means they have to charge, to make a living. Those that don’t charge are not serious Influencers, and most chances they won’t put their heart and knowledge into your project, and are not as committed to your success.
In my opinion, paying an Influencer even more that they ask for, results in more of a desire on their part for your campaign to succeed!
SMI* = Social Media Influencers