Tipping for Makeup Trial?

I am having my makeup trial on Saturday . She is my coworker, she’s really into make up and I’ve always love the way she does her own make up . ( she did go to beauty school ) so I asked her if she would do my make up for the big day and she was so happy and said yes and she said she will not charge me at all!! I do plan to get her a thank you gift .

But now for the trial I want to tip her for sure since she won’t be charging me .

What would be a decent amount $$ to tip her for this make up trial?

thanks