In summary
Feedback from co-workers should be free
If the feedback people share will directly improve something they use, feedback can be free
If the feedback people share has a more indirect impacted on the person who shares the feedback it should be paid.
My two cents
Monica Ray Scott asked and answered this question on Linkedin and I highly recommend that you check out here answer.
What I find interesting in her pieces is this idea that when you give feedback to a service you already use a lot, that service could pay you back not so much with money but with additional gifts linked to the service. If there is a loyalty program, you will get more points. Why not upgrade to the higher class if it's a rail or air transport company?
I have to admit that's something I haven't done that much myself, and I'll try to implement this more in the future. Because it's smart to give something back to someone willing to invest even more in the relationship with your service.
But to come back at the question "Should feedback be free?" my opinion is different depending on the type of feedback:
Work feedback: yes, it should be free. It's your job to also help colleagues
Direct impact feedback: No, if the feedback you give will have a visible impact in the upcoming days or weeks. Then that impact is for me the payment. For example, when I ask someone what they are looking for when they join my newsletter (feedback), I then give them a few links to reach their goal they shared in their feedback (payment).
Indirect impact feedback: Yes, if you will not directly see the impact of your feedback in the upcoming days or weeks. For example, when you rate the quality of a customer support interaction. Yes maybe in a year it will change how customer support is done, but you won't directly feel that your feedback changed this.