Maternity pic question...

TJ1979

My BFF from high school is a professional photographer and has offered to do some maternity pics for me this weekend.  She also did my bridal portraits and they were amazing.  She refused any sort of payment for the bridal pics.  We have not discussed a payment of any kind for the maternity pics, but I don't want her to feel like I'm taking advantage of her.  What is a reasonable amount to stick into a thank you note and give her after the pics?  If I try to just hand her money she won't take it.  I don't want to offend her, but I don't want to take advantage either.  What would you do?  TIA!