Mini Maternity Photo Shoot! How much?

I’m currently a surrogate and want to do a mini maternity session (about 5 photos) because I want to be able to look back on this incredible journey. A photographer said she loved the idea of this and as long as I’m okay with her using these photos on social media, she didn’t want to charge me for the session but instead let me tip whatever I felt like it. I have never gone to her before (so I’m not sure what she normally charges) but she’s professional & takes great quality pictures from what I have seen. What would you tip for this type of session?